The campaigns have been trying to move on from the conflict — which climaxed Tuesday night when each accused the other of calling them a “liar” on the stage immediately following the debate —and are largely not responding to media questions about the rift. The Social Security issue provides a potential opportunity for a liberal tag-team against Biden as both senators have long fought to expand the program and have rolled out plans on the campaign trail.

Biden’s proposal on the campaign trail now calls for expanding Social Security but that is a shift from his days in the Senate when he and other moderate Democrats were open to reforms that would have cut the program in order to help balance the budget. The Biden campaign declined to comment. The Sanders campaign said it had nothing to add to the senator’s previous comments.

Sanders and top campaign aides have been aggressively trying to draw that contrast in recent days. "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. Campaign Manager Faiz Shakir was more pointed, issuing 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."

Biden initially resisted responding to the attacks until Saturday when he accused Sanders‘ campaign of pushing a “doctored video” of him talking about supporting Paul Ryan’s Social Security cuts. In a recent email, the Sanders campaign did say that “Biden lauded Paul Ryan for proposing cuts to Social Security and Medicare” — a claim which PolitiFact labeled “false.” There is no evidence, however, that the Sanders campaign altered any video. In an email to supporters Sunday, the Biden campaign said the Sanders campaign was “pushing around a deceptively edited video.”

Sanders tried to argue Sunday that the 2018 video was distracting from the larger disagreement between the two that stretched decades in Congress. “I think there was a focus on one particular video,” Sanders told The Washington Post . “We should have the whole context.”

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Warren’s comments come hours after she seemed to shy away from a fight with Biden, merely pointing to her own plan to increase monthly payments by $200 and telling reporters “I think everyone’s record on Social Security is important.” Those comments are in line with her usual strategy of not directly criticizing other Democrats but her brief interview this afternoon suggests she may change it up on the issue — which she has highlighted in television ads in Iowa .

Just before she got in a van with campaign aides Sunday, she added: “I'm clear where I am on this, and I think everyone's records should be examined.”

Holly Otterbein contributed to this report