The tit-for-tat over avoiding the fiscal cliff slogged onward Sunday, as Senate leaders remained essentially stuck following a frantic day of horse-trading and bitter attacks by both Democrats and Republicans. 
 
 (PHOTOS: 11 side effects of going over the cliff) 
 
 Both sides now have 48 hours to resolve their differences, or risk a double whammy of historic tax hikes and spending cuts that will make them even more unpopular outside-the-Beltway. 
 
 After every breakthrough, there seemed to be a new obstacle standing in the way of a deal, underlining the perception that the nation’s lawmakers and the White House simply can’t come to terms with each other. Vice President Joe Biden — who had remained on the sidelines for much of the negotiations — returned to Washington on Sunday after Senate Minority Leader Mitch McConnell (R-Ky.) requested it. 
 
 “Mr. President, I rise today frustrated, embarrassed and angry,” said West Virginia Democratic Sen. Joe Manchin on the floor Sunday. “It is absolutely inexcusable that all of us find ourselves in this place at this time, standing on the floor of the Senate in front of the American people hours before we plunge off the fiscal cliff. With no plan and no apparent hope.” 
 
 After loud Democratic protests on Sunday, Republicans agreed to take off the table a controversial provision that would have cut Social Security benefits. But more hurdles emerged, including over automatic spending cuts set to take place next year, and the rates for estate taxes that are set to balloon if no deal is reached by the new year. 
 
 (PHOTOS: Fiscal cliff's key players) 
 
 With no deal at hand, Senate leaders were preparing alternative plans to avoid the cliff’s full impact, including a fall-back option floated by Democrats to force through an extension of current tax rates for families who make less than $250,000, as well as new spending measures to extend jobless benefits for two million unemployed Americans. Republicans were still weighing whether they’d demand 60 votes for passage of even a limited measure, though the prospects for such a bill in the GOP-controlled House remained bleak. 
 
 The House Republican leadership moved Sunday night to give itself expedited authority to bring legislation to the floor in case there was an opening. 
 
 There is also clearly growing tensions between McConnell and Senate Majority Leader Harry Reid (D-Nev.), the pair who took over the fiscal cliff talks on Friday following weeks of fruitless discussions between President Barack Obama and Speaker John Boehner (R-Ohio). 
 
 Democrats are angry at McConnell, believing he’s adopted the “majority of majority” standard Boehner has demanded in the House for the Senate. McConnell does not believe Reid is playing straight with him, saying the Nevada Democrats has shown little urgency by dragging his feet through the weekend of talks. 
 
 McConnell even called in Biden to help break the logjam, although the vice president has offered nothing different from Reid’s position. 
 
 “There is still significant distance between the two sides, but negotiations continue,” Reid said Sunday evening, announcing that the chamber would return to session Monday morning. 
 
 On Sunday afternoon, a new sticking point emerged over “turning off” the sequester, the tens of billions of dollars in spending cuts slated to hit the Pentagon and federal agencies beginning Jan. 2. Democrats want to use some of the revenue from the increased taxes of a cliff deal to postpone those cuts. Republicans, however, are objecting to that proposal, saying they want savings from other areas. 
 
 Democrats have floated a potential compromise to use a portion of the new revenue and other savings from a cliff deal — a “50/50 split” — to “turn off” the sequester, but it is unclear if GOP leaders will agree to that offer. 
 
 And Biden’s inclusion in the negotiations — similar to the role he played in previous legislative fights involving McConnell, an old Senate colleague — sets up a “good cop, bad cop” scenario for Democrats, with Reid playing the heavy and Biden able to work more closely with GOP leaders on Capitol Hill. ||||| Rumored details of President Obama’s proposed budget sparked a firestorm among progressives on Friday. The 2013 budget, expected to be released next week, is said to include phased-in cuts to Social Security benefits. 
 
 “You can’t call yourself a Democrat and support Social Security benefit cuts,” Progressive Change Campaign Committee co-founder Stephanie Taylor said in a widely circulated statement. “The president is proposing to steal thousands of dollars from grandparents and veterans by cutting cost-of-living adjustments, and any congressional Democrat who votes for such a plan should be ready for a primary challenge.” 
 
 The cuts would come in the form of a policy called chained CPI, which was originally proposed by the Republican Party and later adopted by the White House. Chained CPI would reduce the rate at which Social Security benefits increase, effectively cutting the program in the long term. 
 
 Progressive members of Congress emphatically rejected the possibility of passing a budget with chained CPI. 
 
 “In 2008, candidate Barack Obama told the American people that he would not cut Social Security,” said Senator Bernie Sanders, I-Vt. in a statement. “Having him go back on his word will only add to the rampant political cynicism that our country is experiencing today.” 
 
 “I support President Obama, but believe it is deeply misguided and harmful to cut Social Security benefits for seniors, veterans, and children already struggling to get by,” said House Progressive Caucus co-chair Keith Ellison, D-Minn., in a Facebook post. “We should not try to lure Republicans with a policy that harms our seniors, particularly when they have been unwilling to take even a dime away from tax loopholes for millionaires and wealthy corporations.” 
 
 Unions were also unhappy. In a Friday statement on the most recent jobs report, AFL-CIO president Richard Trumka dismissed chained CPI advocacy “as more ‘Washington speak.’” 
 
 President Obama’s support for chained CPI is nothing new. The president included the policy in his proposed fiscal cliff bargain. Though some of the largest labor unions strongly opposed the policy at the time, they declined to withdraw their support for the deal. 
 
 In reaction to the president’s latest rumored proposals, House Speaker John Boehner said that “modest” cuts to entitlements must not “be held hostage for more tax hikes.” In February, multiple Republican strategists told the Washington Post’s Ezra Klein that Obama would need to embrace chained CPI in order to strike a deal with the Republicans—even though, by that point, he already had. In other words, the president has yet to get credit for the concession from Republicans, while taking plenty of heat for it from his own party.
– With the fiscal cliff in sight, Democrats and Republicans are no longer talking. Senate Majority Leader Harry Reid and Minority Leader Mitch McConnell reached a "major setback" today over a fairly wonky provision known as "chained CPI," Democratic sources tell Politico. A means of calculating entitlement benefits, CPI is anathema to many liberals because it would give Social Security beneficiaries lower payments. And Democrats say they have made other concessions, like raising taxes only for those earning more than $400,000. McConnell spoke on the Senate floor today, saying Republicans hadn't received a response to their offer from last night, the New York Times reports. Reid responded that Democrats had no counter-offer because they were "apart on some pretty big issues." President Obama said today on Meet the Press that he would accept "chained CPI" but only as part of a bigger deal that includes at least $1.2 trillion in new revenue over a decade. As it stands, Reid and McConnell are cutting a smaller deal to head off certain tax hikes and Medicare cuts, without tackling the deficit or preventing $109 billion in defense and domestic spending cuts slated for this year.