Eleventh-hour attacks from Bernie Sanders's campaign on Joe Biden's history with Social Security are part of a strategy to undermine the former vice president’s leading argument for his candidacy: that he has the instincts voters are looking for in a post-Trump era.

“It’s judgment. It’s like the Iraq War, where now they all say it was a mistake. But the question is, 'Who said it at the time?'” said Nancy Altman, president of Social Security Works and an expert on the program’s history. “Sanders was against cuts back when Biden was for them. That’s pretty similar to the Iraq War — ‘Hey, I got the judgment.’”

Vermont Sen. Sanders, 78, started the month ripping Biden’s 2002 vote in favor of the Iraq War, which Sanders opposed as a House member, and Biden's support for the effort long after. The issue is bubbling at a critical time in the Democratic presidential primary with just two weeks until the first-in-the-nation Iowa caucuses.

Sanders's presidential campaign in recent weeks has showcased Biden’s opposition to expanding Social Security and entitlements through his 36 years in the Senate and eight years as vice president.

"When I argued that we should freeze federal spending, I meant Social Security as well. I meant Medicare and Medicaid, I meant veterans' benefits. I meant every single, solitary thing in the government,” Biden said on the Senate floor in 1995. A 1983 newspaper clip said Biden indicated support for raising the retirement age.

The tension between the two ideological rivals is bubbling to the surface in their campaigns.

Biden on Saturday called the attacks “a flat lie,” noting that PolitiFact found that the Sanders campaign took one of Biden’s comments out of context. In a 2018 video, Biden mocked former Republican House Speaker Paul Ryan’s plan to cut entitlement spending, not “lauded” it, as the Sanders campaign said.

A fundraising email on Sunday punctuated Biden’s exasperation with the attacks.

“As Democrats, I thought we all knew this election was too important to attack other Democrats. But Bernie Sanders and his campaign don’t care about that,” the email from Biden said. “They’ve decided to unleash a barrage of negative attacks lying about and distorting my record.”

Democrats, including Biden, are now generally unified now in wanting to expand Social Security, with disagreements coming down to which tactics are correct. But that is a recent shift from decades of Democrats playing defense and working on Republicans’ terms fighting against cuts.

“Biden’s the one who keeps talking about compromise with the Republicans, and compromise means a bipartisan solution that has cuts in it,” Altman said.

Biden was part of the Democratic wing that long sought to work with Republicans on Social Security solvency before the party’s shift back to an expansion stance.

During his first term, President Barack Obama, along with then-Vice President Biden, tried to negotiate a budget deal that would trim cost-of-living adjustments along with a tax increase. The plan fell through due to Republican opposition to raising taxes.

Sanders is largely credited for pushing the Democrats to unify on expansion. In the 2016 Democratic primary, he pushed Hillary Clinton leftward, and she came out in support of expanding Social Security after, in 2008, opposing raising taxes to make up for its shortfall.

The 2016 Democratic Party platform explicitly called to expand Social Security, including by opposing raising the retirement age and reductions to cost-of-living adjustments. It was a stark evolution from the party’s 2012 platform that called to protect the program while opposing privatization and “approaches that insist cutting benefits is the only answer.”

While Biden now campaigns with a plan to expand some Social Security benefits, Sanders’s attacks could give voters pause when considering voting for the former vice president if they associate him with an old Democratic Party establishment that voters rejected in the 2016 general election. And polls show overwhelming support for the entitlement program.

"Tea Party-ers agree with union members," Altman said. "As polarized as we are, we're not about this issue. People are very clear: They don't want it cut, and they certainly don’t want it privatized or radically changed."