WASHINGTON (AP) - A month into the rollout of President Barack Obama's health care overhaul and no end to problems, the senior administration official closest to the law's implementation will answer questions Tuesday from a Senate panel that wrote much of it. Medicare chief Marilyn Tavenner will go before the Health, Education, Labor and Pensions, or HELP, committee as the balky HealthCare.gov website continues to experience service problems. Adding to website woes is the political fallout from a wave of cancellation notices reaching millions of consumers who currently buy individual policies. Those plans don't meet requirements of the law taking effect next year. Republicans had seemingly reached a dead end in their drive to repeal "Obamacare," but they're now relishing new lines of attack and a technology fiasco handed to them by the administration. Democrats are uneasy. While hoping that the administration can deliver its latest promise that the website will be running smoothly for most people by the end of November, they acknowledge there's no clear evidence yet that will happen. The website went down again in the middle of the day Monday for about 90 minutes. And the administration still refuses to divulge enrollment statistics until mid-November. Less well known than Health and Human Services Secretary Kathleen Sebelius, Tavenner was closer to the day-to-day work of setting up the enrollment website, handled by experts within her agency, the Centers for Medicare and Medicaid Services, and outside contractors. Through the summer, she assured Congress that everything was on track for a reasonably smooth launch of new health insurance markets in all 50 states. Testifying before the House Ways and Means committee last week, Tavenner said that had been her firm conviction. While some planned functionalities were postponed as the launch date neared, she still expected the site to function more or less well. HealthCare.gov is not only the portal for average consumers, it serves also as a conduit used by trained enrollment counselors and the government contractors who process telephone and paper applications. The Democratic-led Senate is friendly to the Affordable Care Act, a bulwark against the repeal drive emanating from the Republican-run House. Although the law is usually described as Obama's signature legislation, it is also a political legacy item for many senior Democratic senators. HELP chairman Tom Harkin, an Iowa Democrat, appealed for calm on Monday. "We're missing the big picture," Harkin said on MSNBC. Yes, we had problems with the rollout. That's inexcusable, but that's being fixed ... millions and millions of Americans are getting better health care than they had before. Tavenner began her career as a nurse and built a successful record as a hospital executive before entering public service. Seen as a businesslike manager, she has enjoyed support from lawmakers across the political spectrum. HealthCare.gov was supposed to provide one-stop shopping for people who don't have a health plan on the job. Its target audience is not only uninsured Americans but those who already purchase coverage individually. Middle-class people can sign up for private coverage made more affordable by tax credits that act like a discount on premiums. Lower-income people will be steered to an expanded version of Medicaid in states that agreed to expand that safety net program. A new study released Tuesday estimates the potential size of the market nationally at 28.6 million people. The nonpartisan Kaiser Family Foundation says three out of five, or more than 17 million people, will be eligible for tax credits. That includes both uninsured people and those switching over from current individual plans. Texas, California and Florida have the highest numbers of residents eligible for the credits. Earlier, the Congressional Budget Office estimated that 7 million uninsured people would gain coverage through the marketplaces, a statistic the Obama administration adopted as its own enrollment target.