“Bernie was key in that evolution,” said Jared Bernstein, a senior fellow at the left-leaning Center on Budget and Policy Priorities, who served as Biden’s chief economist and economic adviser from 2009 to 2011.

It was in December 2010 that Sanders filibustered the Biden-negotiated deal with Republicans that extended Bush-era tax cuts, cut the estate tax, continued unemployment benefits and created a temporary Social Security payroll tax cut, or “tax holiday.”

The deal was struck just a month after Republicans, in the words of President Obama, had just “shellacked” Democrats at the polls. Obama and Biden were facing reelection in less than two years; the president wanted to position himself as a reasonable centrist who was serious about deficit reduction. He was mindful he had to deal with divided government in Congress, and feeling the pressure to stimulate the economy with tax cuts because the scars of the recession were not healed.

After Biden briefed the Democratic caucus about the plan, Sanders was angered, said his senior adviser Warren Gunnels: “You can see Bernie sitting there listening to that and his blood starting to boil.”

Sanders said he believed the only concession Democrats received was the jobless benefits extension — but his team had determined that that such benefits had been extended on a bipartisan basis every time the unemployment rate was as high as it was at the time. And he fretted that temporarily trimming Social Security taxes could ultimately help weaken the program.

So Sanders filibustered, mentioning Biden three times only by his title — but never saying his name — on the Senate floor as he blasted the deal the then-vice president negotiated with Republicans.

“I know the vice president recently made the point this was originally a Republican idea,” Sanders said, referring to the caucus meeting. “Why did the Republicans come up with this idea? These are exactly the same people who do not believe in Social Security.”

Biden responded with an op-ed in USA Today a month later where he accused opponents of the deal, without naming Sanders, of spreading “misinformation.”

Sanders failed to stop the measure. And his specific fears about weakening Social Security’s tax structure didn’t come to pass, though Social Security advocates said it helped popularize the idea of payroll-tax holidays.

But it catapulted Sanders into the burgeoning progressive movement’s national spotlight by criticizing Obama and Biden from the left. His speech trended on Twitter and blanketed cable television news, and Sanders’ office was inundated with more calls than they had ever received, an aide said.

“Bernie won the debate outside the Beltway,” Gunnels said. “I really think that lit a spark for the movement that Senator Sanders has led to create. That was really kind of the launching pad.”

Bernstein, Biden’s former adviser, agreed, saying Sanders was “showing himself to dissent from the mainstream Democratic playbook, trying to break from a fold that he felt was not pursuing good politics or policy. There’s clearly been a constituency of Democrats who are seeking a more liberal or progressive set of policies than the mainstream was serving up in those years.”

“That group has gotten a lot more attention from candidates and even the more moderate candidates have to their credit evolved with this constituency,” Bernstein said.

Sanders supporters chuckle at the irony that Sanders’ national political profile — which is helping him mount one of the biggest threats to Biden’s presidential ambitions — is, in part, the inadvertent byproduct of Biden’s penchant for cutting deals with Republicans.

Bernstein cautions that, while there’s a “leftward drift in the evolution of our agenda” that Sanders helped harness, Democratic voters are still moderate and Biden is a “full-throated and full-blooded supporter of Social Security.”

While Biden’s campaign won’t discuss his previous support for Social Security cuts and spending freezes, his defenders acknowledge he changed. They say the major reasons for that weren’t due to Sanders, but the bad economy of the recession, the bad faith of Tea Party Republicans who dictated the congressional agenda after winning the House in 2010, and a new perception in Washington political circles that didn’t view deficits, especially from safety-net programs, as nearly as dangerous as they previously had.

Biden’s earliest forays into Social Security cuts happened in 1984 , when congressional concerns over Reagan-era deficits grew and he co-sponsored a failed proposal with Republicans to freeze spending, including cost-of-living adjustments for Social Security, which would have had the effect of a financial cut for recipients and a savings for government. Reagan and the Senate Republican leadership opposed it, saying it contained serious cuts to Social Security.

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Sanders’ campaign pointed out last week that Biden in 1995 recalled on the Senate floor his four prior attempts to balance the budget, saying that he had been ready “to freeze all government spending, including Social Security, including everything.”

Before his second failed bid for president, Biden in 2007 said he would “absolutely” consider Social Security and Medicare spending limits but noted the political risks in pursuit of balancing the budget.

“The political advisers say to me is ‘whoa, don’t touch that third [rail],’” Biden said in a 2007 “Meet the Press” interview. “Look: the American people aren’t stupid.”

During the 2012 reelection, Biden struck a different posture when he told voters in Virginia that “I guarantee you, flat guarantee you, there will be no changes in Social Security. I flat guarantee you.” But after reelection, the Obama administration, locked in negotiations with Republicans over what was known as the “fiscal cliff,” offered to change the way cost-of-living adjustments are calculated. Ultimately, the plan went nowhere because Republicans refused to consider tax increases and progressive Democrats balked, led partly by Sanders, at the Social Security plans.

Biden’s campaign accused Sanders and his team of failing to account for the liberal work the former vice president and president did for safety-net programs by passing Obamacare, the 2009 stimulus bill and the 2010 tax cut package that included the Social Security “tax holiday” that was deemed effective by some economists and his team said was a “godsend for working Americans hit indescribably hard by the financial crisis.”

Biden’s campaign pointed to a 1983 vote he cast that helped keep the program from looming insolvency and said that, years later, the Obama-Biden administration opposed using Social Security or Medicare for deficit reduction.

“Vice President Biden is proud that he helped create this economic lifeline for struggling families. Between this and the Recovery Act, the vice president was critical to ending the second-worst recession in American history,” Biden spokesman Andrew Bates said. “By contrast, Senator Sanders’ approach would have threatened another recession in the pursuit of purism ... Real leadership in public service is about achieving results for people who need help."

Phil Schiliro, the head of the Obama White House’s legislative affairs from 2009-2011, said of Biden that “I never saw him advocate for safety net cuts. The vice president’s positions always reflected what the president’s positions were.”

To progressives, though, Sanders was a voice in the wilderness standing up for Social Security during that time.

“It was a very lonely place,” said Alex Lawson, executive director of Social Security Works and a Sanders supporter. “There was a cacophony of calls for doing something about the debt and Bernie Sanders basically rejected the premise that that’s what it was about...In 2010, the ‘grand bargain’ was at the peak of its popularity in the elite media, in the elite political circles in D.C., in this reinforcing bubble that decided that it made a lot of sense to cut benefits.”

