After spending a week mired in a back-and-forth with fellow progressive Elizabeth Warren , Sanders on Saturday came under some welcome fire from Joe Biden over the tactics used by his camp to criticize the former vice president's record on Social Security.

The clash comes less than two weeks before the Iowa caucuses, with the latest polls showing Sanders, Biden, Warren, and Pete Buttigieg all within striking distance of victory -- and one another. The Sanders campaign sought out direct conflict with Biden throughout the campaign, but the candidate and his supporters sharpened their attacks over the last 24 hours as they worked to highlight the contrasts between Sanders' expansive, progressive vision and Biden's pledge to restore normalcy in Washington by seeking a measure of reconciliation with Republicans.

Biden's instinct for deal-making, often to the frustration of liberals, is now under renewed scrutiny as part of that debate. At an event in Iowa, he accused "Bernie's people" of promoting a "doctored video ... saying that I agreed with Paul Ryan, the former vice presidential candidate, about wanting to privatize Social Security." The claim set off its own debate over the veracity of the clip, which was taken from Biden's 2018 speech at the Brookings Institution. Though the video takes Biden's remarks that day out of context, it has not been altered.

But questions over the video, which has been shared by senior Sanders aides, have now given way to a broader clash -- which the Sanders campaign has been pushing for weeks but Sanders himself did not bring up during last week's debate -- over Biden's past willingness to entertain negotiations with Republicans over deals that would either slow the program's growth or reduce its scope.

Speaking to reporters on Sunday in New Hampshire, Sanders dug in on the charges.

"I think anyone who looks at the vice president's record understands that time after time after time, Joe has talked about the need to cut Social Security," Sanders said. "That's in the congressional record. He has said that many, many times. I don't think that's disputable."

Sanders specifically cited Biden's role in "the so-called 'Balanced Budget' effort," a mid-1990s push to require by law that government spending does not exceed revenue in a given fiscal year, before highlighting his opposition: "I put together a -- we called it a 'defending Social Security caucus,' (and) played a very active role in saying, 'No, you can't cut Social Security when for millions of elderly people, this is what they live on.' You can't cut it."

"He's said it many, many times," Sanders added, "that he believes it appropriate to cut social security or freeze (cost-of-living adjustments) or to raise the retirement age. That's just the record. I don't think anyone can debate that."

The questions over Biden's speech, though, muddled the campaign's attempt to bring the fundamental issue to the foreground.

Sanders campaign speechwriter David Sirota previously cited Biden's Brookings speech in a newsletter to supporters, writing that Biden "lauded Paul Ryan for proposing cuts to Social Security and Medicare." PolitiFact rated that description as "false," writing that Sirota had misrepresented Biden's "sarcastic" comment about Ryan.

Social Security is one of the most popular government programs across party lines, along with Medicare, and many high-ranking Democrats in the 1980s and 1990s -- like Biden -- sought to signal their seriousness about the deficit by expressing a willingness to negotiate with the GOP over plans to slow its growth or raise the retirement age. Despite having gone on the record in the past decades saying he would be willing to buck his party to strike a bipartisan deal on Social Security, Biden drew a clear line on one of the most aggressive GOP proposals: The George W. Bush administration quickly abandoned a 2005 move to privatize it.

Biden spokesman Andrew Bates on Sunday pushed back against the full force of the Sanders campaign and its supporters, who have flooded social media with an assortment of old news clips showing Biden talking about potential changes to the program, citing Sanders' own words.

"As Bernie Sanders himself said in 2015: 'Joe Biden is a man who has devoted his entire life to public service and to the wellbeing of working families and the middle class,'" Bates said in a statement.

As Biden's claim about the video began to circulate on Saturday evening, Sanders campaign manager Faiz Shakir fired back, saying in a statement that Biden "should be honest with voters and stop trying to doctor his own public record of consistently and repeatedly trying to cut Social Security."

"The facts are very clear: Biden not only pushed to cut Social Security -- he is on tape proudly bragging about it on multiple occasions," Shakir said. Biden during his time in the Senate publicly expressed an openness to either freezing year-over-year cost-of-living increases, as part of a bipartisan plan to slow federal spending, or raising the retirement age in order to preserve the program while managing the federal budget.

Sanders has been a consistent and fierce supporter of growing the New Deal-era program and, during the Obama years, emerged as a leading opposition voice when it was reported that cuts were being considered by a bipartisan presidential deficit commission. Last year, the Vermont senator reintroduced legislation that would add more than 50 years of sustainability to the program , which on its current track would no longer be able to pay out full benefits by 2034, by subjecting all income above $250,000 to the existing 12.4% Social Security payroll tax, which is split between workers and employers. Sanders would also levy a new 6.2% tax on investment income above $200,000 for individuals and $250,000 for couples.

The revenue boosts proposed by Sanders, in the legislation and by his campaign, would increase benefits for all Social Security recipients and make the annual cost of living adjustment more generous.

In an interview with CNN's Anderson Cooper earlier this month, Sanders skewered Biden on everything from his support for NAFTA, the unpopular and soon-to-be-replaced trade deal, to his vote to authorize the Iraq War.

"Joe Biden has been on the floor of the Senate talking about the need to cut Social Security or Medicare or Medicaid," Sanders added, as part of a riff suggesting that his Democratic primary rival would make for a poor candidate in November's general election.

A Biden campaign aide pointed to the former Delaware senator's "100% lifetime record on Social Security votes" from the Alliance for Retired Americans as evidence of his fidelity to the program and accused the Sanders campaign of pushing a "video and transcript that were intentionally, deceptively edited to make it seem like Vice President Biden was praising and agreeing with Paul Ryan, when it is clear he was doing the exact opposite." The aide argued that the clip met the threshold for being labeled as "doctored."

Despite the questions over how they had initially presented their case against Biden, the Sanders campaign seemed to delight in Biden stirring the pot on an issue they believe Biden's record -- when cast against Sanders' -- could damage him with Democratic primary voters with the first round of voting, in Iowa, a little more than two weeks away.

"Biden is desperately trying to muddy the waters by dishonestly manufacturing a fake nonsensical controversy over one single video -- because he knows there are multiple videos out there of public speeches he made proudly congratulating himself for working to cut Social Security," Sirota told CNN. "In trying to cut Social Security, Joe Biden is telling us exactly who he is -- and we should believe him."