(CNN) A federal judge in Texas said on Friday that the Affordable Care Act's individual coverage mandate is unconstitutional and that the rest of the law therefore cannot stand. 
 
 Legal experts say the ruling won't immediately affect Americans' health coverage, and a group of states led by California is already vowing to appeal. But the invalidation of the landmark health care law popularly known as Obamacare throws into doubt the future of health coverage for millions of Americans on the Obamacare exchanges and in Medicaid expansion. 
 
 The ruling and expected appeal sets up another cliffhanger in which the fate of the law, which Republicans have unsuccessfully tried to repeal for years, will likely once again ultimately lie with the Supreme Court. 
 
 Chief Justice John Roberts cast the deciding vote leaving the law intact in 2012, but it's unclear how the justices will view this challenge, which centers on changes to the individual mandate that were baked into the 2017 tax reform. 
 
 President Donald Trump, who campaigned in 2016 on a promise to undo Obamacare, was thwarted in Congress last year by a lone vote from the late Arizona Republican Sen. John McCain. 
 
 Trump immediately tweeted in celebration on Friday and called on Congress to act. 
 
 "As I predicted all along, Obamacare has been struck down as an UNCONSTITUTIONAL disaster! Now Congress must pass a STRONG law that provides GREAT healthcare and protects pre-existing conditions. Mitch and Nancy, get it done!" Trump wrote. 
 
 He later added, "Wow, but not surprisingly, ObamaCare was just ruled UNCONSTITUTIONAL by a highly respected judge in Texas. Great news for America!" 
 
 On Saturday, Trump told reporters that if the decision is eventually upheld by the Supreme Court, "we will be sitting down with the Democrats" to "get great health care for our people." 
 
 "Let's say repeal and replace, handled a little bit differently, but it was a big, big victory by a highly respected judge," Trump said during a visit to Arlington National Cemetery. 
 
 Later Saturday, Obama took to Facebook to urge people to enroll in the program, writing in a post that Friday's ruling "can be a scary thing to hear, particularly if you or someone you care about has a pre-existing condition." 
 
 "As this decision makes its way through the courts, which will take months, if not years, the law remains in place and will likely stay that way," he wrote. 
 
 Democratic House leader Nancy Pelosi decried the ruling and suggested that the coming Democratic-led House will intervene in the case. 
 
 "Republicans are fully responsible for this cruel decision and for the fear they have struck into millions of families across America who are now in danger of losing their health coverage," Pelosi said in a statement released late Friday. "When House Democrats take the gavel, the House of Representatives will move swiftly to formally intervene in the appeals process to uphold the life-saving protections for people with pre-existing conditions and reject Republicans' effort to destroy the Affordable Care Act." 
 
 Today, the law governs American health care. 
 
 More than 4 million people have already signed up for 2019 coverage on the exchanges, and millions more are expected to pick plans before open enrollment ends Saturday. 
 
 The ruling threatens to wipe away the Affordable Care Act's protections for those with pre-existing conditions, which became a focal point of the midterm elections and helped Democrats take the House. 
 
 In his opinion, District Judge Reed O'Connor said the "Individual Mandate can no longer be fairly read as an exercise of Congress's Tax Power and is still impermissible under the Interstate Commerce Clause—meaning the Individual Mandate is unconstitutional." 
 
 He also held that the individual mandate is "essential to and inseverable from the remainder of the ACA." 
 
 The case against the ACA was brought by 20 Republican state attorneys general and governors, as well as two individuals. It revolves around Congress effectively eliminating the individual mandate penalty by reducing it to $0 as part of the 2017 tax cut bill. The mandate requires nearly all Americans to get health insurance or pay a penalty. 
 
 The Republican coalition, led by Texas Attorney General Ken Paxton, is arguing that the change rendered the mandate itself unconstitutional. The states say that the voiding of the penalty, which takes effect next year, removes the legal underpinning the Supreme Court relied upon when it upheld the law in 2012 under Congress' tax power. 
 
 The Trump administration said in June that it would not defend several important provisions of Obamacare in court. It agreed that zeroing out the penalty renders the individual mandate unconstitutional but argued that invalidates only the law's protections of those with pre-existing conditions. These include banning insurers from denying people policies or charging them more based on their medical histories, as well as limiting coverage of the treatment they need. 
 
 But the administration maintained those parts of the law were severable and the rest of the Affordable Care Act could remain in place. 
 
 Professor Tim Jost, of Washington and Lee University, noted that O'Connor went further than the Trump administration had asked. 
 
 "The Trump administration only asked that the individual mandate and provisions protecting individuals with pre-existing conditions be invalidated, but O'Connor's order would invalidate many provisions of the Medicaid program, the Medicare program and other federal laws," he told CNN. 
 
 But Jost said the structure of the challenge and of O'Connor's ruling means that nothing will change right away. 
 
 "Judge O'Connor has declared the individual mandate unconstitutional and the rest of the Affordable Care Act invalid, but he has not blocked its continued operation," Jost said. 
 
 Because the administration would not defend the law, California, joined by 16 other Democratic states, stepped in. They argued that the mandate remains constitutional and that the rest of the law, in any event, can stand without it. Also, they said that eliminating Obamacare or the protections for those with pre-existing conditions would harm millions of Americans. 
 
 Appeal coming 
 
 California Attorney General Xavier Becerra called the ruling "an assault" and a spokesperson for his office indicated an appeal would be coming. 
 
 "Today's ruling is an assault on 133 million Americans with preexisting conditions, on the 20 million Americans who rely on the ACA's consumer protections for healthcare, on America's faithful progress toward affordable healthcare for all Americans," Becerra said in a statement. "The ACA has already survived more than 70 unsuccessful repeal attempts and withstood scrutiny in the Supreme Court." 
 
 American Medical Association president Barbara McAneny said the group will work with other groups in pursuing an appeal and reversal of the decision. 
 
 "Today's decision is an unfortunate step backward for our health system that is contrary to overwhelming public sentiment to preserve pre-existing condition protections and other policies that have extended health insurance coverage to millions of Americans," she said in a statement. "It will destabilize health insurance coverage by rolling back federal policy to 2009. No one wants to go back to the days of 20 percent of the population uninsured and fewer patient protections, but this decision will move us in that direction." 
 
 Meanwhile, Paxton applauded the ruling and said it represents a massive check on federal power. 
 
 "Today's ruling enjoining Obamacare halts an unconstitutional exertion of federal power over the American health care system while our multistate coalition lawsuit works its way through the courts," Paxton said in a staement. "Our lawsuit seeks to effectively repeal Obamacare, which will give President Trump and Congress the opportunity to replace the failed social experiment with a plan that ensures Texans and all Americans will again have greater choice about what health coverage they need and who will be their doctor." 
 
 Steve Vladeck, a CNN legal analyst and a law professor at the University of Texas School of Law, said he felt an appeal could be looked on favorably by the Supreme Court. 
 
 "The ruling relies on conclusions about who can bring this suit and about the impact of President Trump's tax bill that have been widely debunked by scholars from across the political spectrum," Vladeck said. "It will certainly generate headlines, and it may even be upheld by the conservative federal appeals court in Texas, but it's difficult to imagine that the Supreme Court—which has already turned away two challenges to the ACA—would agree with tonight's ruling." 
 
 Pre-existing conditions and the midterm fight 
 
 The lawsuit entered the spotlight during the midterm elections, helping propel many Democratic candidates to victory. Protecting those with pre-existing conditions became a central focus of the races. Some 58% of Americans said they trust Democrats more to continue the law's provisions, compared to 26% who chose Republicans, according to a Kaiser Family Foundation election tracking poll released in mid-October. 
 
 The consumer protections targeted by the administration are central to Obamacare and transformed the health insurance landscape. Their popularity is one of the main reasons GOP lawmakers had such difficulty repealing Obamacare last year. 
 
 "Guaranteed issue" requires insurers to offer coverage to everyone regardless of their medical history. Prior to the Affordable Care Act, insurers often rejected applicants who are or had been ill or offered them only limited coverage with high rates. 
 
 Under the law's community rating provision, insurers are not allowed to set premiums based on a person's health history. And the ban on excluding pre-existing conditions from coverage meant that insurers cannot refuse to pay for treatments because of a policyholder's medical background. 
 
 All these provisions meant millions of people with less-than-perfect health records could get comprehensive coverage. But they also have pushed up premiums for those who are young and healthy. This group would have likely been able to get less expensive policies that offered fewer benefits prior to Obamacare. That has put the measures in the crosshairs of Republicans seeking to repeal the law and lower premiums. 
 
 It's no wonder that politicians on both sides of the aisle promised to protect those with pre-existing conditions during the election. Three-quarters of Americans say that it is "very important" for the law to continue prohibiting health insurers from denying coverage because of medical histories, according to the Kaiser Family Foundation's September tracking poll -- 58% of Republicans feel the same way. And about the same share of Americans say it's "very important" that insurers continue to be barred from charging sick people more. ||||| CLOSE A new Reuters/Ipsos poll shows that nearly 60 percent of likely voters across the nation want the Affordable Care Act to remain in place. Buzz60 
 
 Hundreds of people march through downtown Los Angeles protesting President Donald Trump's plan to dismantle the Affordable Care Act, his predecessor's signature health care law, March 23, 2017. (Photo: Reed Saxon, AP) 
 
 On Friday, a federal judge fulfilled a wish conservatives have held for more than eight years by ruling the Affordable Care Act, also known as Obamacare, unconstitutional. 
 
 U.S. District Court Judge Reed O’Connor agreed with a group of 20 states with Republican governors or legislatures that argued the constitutionality of the law's individual mandate dissolved when Congress removed the tax penalty for the uninsured. 
 
 In 2012, the Supreme Court upheld the law and its mandate requiring people purchase insurance on the grounds that it fell within Congress' taxation power. O’Connor said when the tax penalty was removed so was the central argument upholding the 2010 law's constitutionality. His ruling declared not only the individual mandate but the entire law unconstitutional. 
 
 That decision leaves Republican and Democratic lawmakers, as well as millions of Americans, wondering, "So, what happens now?" 
 
 For the immediate future, the answer is nothing. The ACA will remain in place while the law's future is handled in the courts, a process could take months or years to resolve. People who bought coverage on the health care exchanges before Saturday's deadline will be insured for 2019. 
 
 President Trump: Ruling against Obamacare offers another chance for repeal and replace 
 
 'Republicans will never stop': Obama slams GOP after ruling against Affordable Care Act 
 
 The legal battle 
 
 The Justice Department under President Donald Trump announced in June that it would not defend ACA's individual mandate and other provisions, such as protections for people with pre-existing conditions, but asked that the law not be struck down in its entirety. 
 
 So, the Justice Department seems unlikely to appeal O'Connor's ruling. But after the Trump administration dropped its defense of the ACA, a number of Democratic states, led by California, took it up. Those states intend to appeal Friday's decision. 
 
 "The ACA has already survived more than 70 unsuccessful repeal attempts and withstood scrutiny in the Supreme Court. Today’s misguided ruling will not deter us: our coalition will continue to fight in court for the health and well-being of all Americans," California Attorney General Xavier Becerra said Friday in response to the ruling. 
 
 House Minority Leader Nancy Pelosi, D-Calif., has vowed that Democrats will intervene and direct House counsel to defend the ACA when they take control of the chamber next month. 
 
 The case would go the U.S. Court of Appeals for the 5th Circuit, which is dominated 11-5 by judges who were nominated by Republican presidents. Yale Law professor Abbe Gluck said the 5th Circuit will almost certainly grant a stay, keeping O'Connor's ruling on hold pending their decision. 
 
 A three-judge panel for the 5th Circuit will then accept briefs and hear oral arguments in the case before giving their decision. From there, it's possible the entire panel of 5th Circuit judges will ask to review the case. 
 
 "You'd be lucky if you had an opinion out of them before June," Gluck said. 
 
 Whatever the 5th Circuit's decision, one side or the other is almost certain to appeal. But Gluck said she believes the "current Supreme Court will not accept the case" if O'Connor's decision is overturned because "the case is extremely weak on the law and I don't think anyone on the court has the appetite for another politicized Obamacare showdown." 
 
 If O'Connor's ruling is upheld, on the other hand, the Supreme Court "will have to take the case." Barring a rare special session to hear the case, Gluck said it is most likely the Court will hear arguments in the case in early October. 
 
 Severability 
 
 Gluck, who called the ruling "shocking" in a New York Times op-ed co-authored with Case Western Reserve University School of Law professor Johnathan Adler, expects the case will be overturned and never even make it the Supreme Court. 
 
 "This case turned on a very simple and established legal principle, which conservative and liberal judges alike apply," she said. That principle, known as "severability" is that when a court rules part of a law unconstitutional, the rest of the law should stand "unless Congress clearly indicates otherwise." 
 
 "Sometimes severability cases are difficult because it is hard to guess how much importance Congress attributed to one provision, especially in a lengthy law like the Affordable Care Act," Gluck and Adler wrote in the Times. "But this is an easy case: It was Congress, not a court, that eliminated the mandate penalty and left the rest of the statute in place. How can a court conclude that Congress never intended the rest of the statute to exist without an operational mandate, when it was the 2017 Congress itself that decided it was fine to eliminate the penalty and leave the rest of the law intact?" 
 
 O'Connor said Republicans in Congress eliminated the tax penalty but left the rest of the law intact because they didn't have the vote for a full repeal. 
 
 Gluck and Adler called that argument "ridiculous" and said Congress' intent has to be determined by how it votes. 
 
 "Congress thinks the law works without an operational mandate. To believe otherwise is to assume Congress enacts unworkable laws and that is not what courts are allowed to presume," they wrote. 
 
 Market effect 
 
 The legal uncertainty around the law could have negative impacts on the health insurance markets, said UCLA Law professor Jill Horwitz. 
 
 "Insurance markets depend on stability," said explained. "That kind of roiling of the waters leads to an uncertain legal landscape that insurance companies don't like to operate in." 
 
 The instability makes it difficult for insurance company executives to plan because they don't know if the exchanges will continue to function or how many people are going to be eligible to shop in them, Horwitz said. 
 
 "The rates are set for next year, but I imagine that this kind of uncertainty only is going to lead to higher prices for the year after," she said. 
 
 "And if it's difficult for experts to follow all the twists and turns of the state of the ACA, imagine how hard it is for somebody who's working a full-time job, trying to live their lives, not an expert in insurance markets, without the help of outreach because we've cut down on the funding for enrollment counselors," Horwitz said. "Imagine how hard it is for those people just to find insurance." 
 
 Even if O'Connor's decision is reversed by the 5th Circuit or Supreme Court, Horwitz said markets could still see price increases for three reasons. 
 
 "One is the insurance companies just don't like to live under that kind of uncertainty and so they're going to price that uncertainty into their products," she said. 
 
 The second risk is that consumers "are going to get the wrong message" and, without a major effort at outreach, fewer of them will go to the insurance marketplaces. 
 
 Third, it could mean that "instead of policy makers spending their time improving the health care system for the people who need it, they're spending their time litigating this case." 
 
 More: Trump administration touts 2019 drop in average Obamacare premiums after big increases this year 
 
 Search for a legislative solution 
 
 The solution offering the most stability and predictability for both insurance company and consumers would be a legislative one, Horwitz said. 
 
 "On the assumption that the Supreme Court upholds, we will get great, great health care for our people," President Donald Trump told reporters Saturday. "We’ll have to sit down with the Democrats to do it, but I’m sure they want to do it also." 
 
 Democrats will undoubtedly want to pass health care legislation, but getting them to agree with Trump and Senate Republicans on the specifics will be a challenge. 
 
 A call for "Medicaid for all" and to expand a public insurance option is popular among Democrats. Health insurance, and retaining coverage for people with pre-existing conditions, was a central campaign issue for Democrats who won a House majority in the midterm election. 
 
 And Democrats believe they have a political advantage in the debate. As the likely incoming Speaker Pelosi will be able to order the House counsel to intervene in the ACA without a vote. But she intends to bring one anyway to get Republicans on record, Politico reported. Schumer said Sunday he plans to push for a similar vote in the Senate, for similar reasons. 
 
 But those House Democrats will need the Republican-controlled Senate to go along with any measure to make it Trump's desk in the next two years and "Medicaid for all" is unlikely to have the same appeal across the aisle. 
 
 Republicans have expressed more willingness to keep provisions of the Affordable Care Act in place since their failed efforts to "repeal and replace" the law and some have suggested a more step-by-step approach. 
 
 For example, keeping insurance for people with pre-existing conditions has been an area where Republicans and Democrats could work together. Other popular provisions such as parents being able to keep their children on their plan until age 26 could also possibly make it through as separate pieces of legislation. 
 
 Sen. Susan Collins, R-Maine, said Sunday that O'Connor's ruling was "far too sweeping" and that she believes "it will be overturned." 
 
 "There are many good provisions of the law. Those should be retained," Collins said. 
 
 Law professor Gluck said she is glad Collins and other lawmakers recognize the weak legal standing of O'Connor's ruling but fear it could lead to inaction. 
 
 "It's good that there's so much agreement among legal experts that the decision will likely be overturned, and I'm glad that message is getting out," she said. "I think there's a danger in that message becoming widespread to the extent that it makes the members of Congress and the White House feel that they can just sit back and do nothing." 
 
 Contributing: Richard Wolf 
 
 More: While Republicans tried to end Obamacare, uninsured rate unchanged in Trump's first year 
 
 More: Donald Trump ‘better’ at managing Obamacare than Obama, health secretary says 
 
 Read or Share this story: https://www.usatoday.com/story/news/politics/2018/12/17/what-next-affordable-care-act/2330715002/
– A Texas judge delivered a bombshell ruling on ObamaCare Friday night, declaring that the Affordable Care Act is unconstitutional. US District Court Judge Reed O'Connor ruled that the individual mandate—the penalty those who forgo insurance must pay—is unconstitutional. And because the individual mandate is so integral to the ACA, the entire ACA is unconstitutional as well, he declared. All of which has lawmakers in both parties, as well as millions of Americans who get their coverage from the ACA, wondering, "Now what?" Here's what we know, including a debate over the legal principle of "severability": What's next: In terms of the immediate future, nothing changes, reports USA Today. Though much uncertainty is now in the mix, the ACA remains in place while this plays out in the courts—perhaps even at the Supreme Court, for the third time—and the process could take months, if not years. Those insured under the ACA, including those who just enrolled, remain covered. The stakes: Here's how the Washington Post puts it: "The opinion, if upheld on appeal, would upend the health insurance industry, the way doctors and hospitals function, and the ability of millions of Americans to access treatments they need to combat serious diseases." Core provisions: As the AP notes, key provisions of the ACA have become popular with Americans, including its protections for pre-existing conditions, expanded Medicaid for lower-income Americans, and the ability of parents to keep their kids covered through age 26. All of which makes the politics tricky for the GOP; the AP says the decision "has landed like a stink bomb among Republicans."