MASON CITY, Ia. — Joe Biden and Bernie Sanders aren't easing up on a dispute about Biden's record on Social Security.

Biden, the former vice president, and Sanders, a U.S. Senator from Vermont, are leading candidates heading into the Feb. 3 Iowa caucuses. With less than two weeks before the first-in-the-nation nominating contest, each is arguing they have the strongest records of protecting Social Security, and that the other side is trying to tear them down.

Biden released a video Tuesday that accused Sanders of unleashing "a barrage of negative attacks" and calling them "dishonest" for portraying him as anything other than a staunch defender of Social Security.

Officials with Sanders' campaign are saying Biden is the first campaign to go negative, when Sanders was just trying to highlight different records on social safety net programs — one where Biden, Sanders' campaign argues, has advocated freezing all federal spending, including social programs, while Sanders has continuously advocated their expansion.

"Bernie’s campaign is not telling the truth," a narrator says in the Biden ad. "Joe Biden has repeatedly voted to save Social Security. He and President Obama beat back Republican attempts to privatize it, and, in 2012, Joe Biden even said he didn’t support those cuts — to Paul Ryan."

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The Sanders ad features audio of Biden saying, in 1995, "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, campaigning in Iowa on Wednesday, reiterated his position on Social Security and argued the clip in Sanders' ad was missing context. In a "Morning Joe" segment filmed Wednesday morning in Mason City, he was asked if he would cut Social Security benefits as president.

"No. No. No. No," Biden said, adding about his 1995 remarks, "And we weren’t talking about cutting them either then. That was trying to figure out how we got through a debacle where the whole government didn’t shut down."

In Des Moines on Wednesday, Sanders' senior adviser Jeff Weaver defended the characterization.

"The vice president is all over YouTube with floor speeches in the United States Senate, with television interviews, talking about the need to freeze Social Security, Medicare, veterans' benefits and bragging about it at the time," Weaver said in an interview with the Des Moines Register. "It's not mischaracterizing. Folks can go and see it for themselves, unfiltered. What we put up last night in response to Vice President Biden's negative attack ad was just that — it was audio of Vice President Biden talking, with great pride, about his efforts to cut Social Security, Medicare, veterans' benefits and other important federal social safety net programs."

Weaver pointed to social programs' vulnerability in times of ballooning deficits and President Donald Trump's remarks Wednesday morning that he's open to cutting social programs.

"You have Donald Trump talking about, if he's reelected, he's open to making cuts," Weaver said. "You have Vice President Biden, who has a long record of advocating for cuts and freezes in Social Security and Medicare.

"And then you have Bernie Sanders, who has consistently and unwaveringly supported the health and expansion of Social Security, Medicare and other programs. So it is an important issue, and I think it will become a more important issue given what Trump has done."

Biden told WHO-TV on Tuesday that Sanders apologized personally for an opinion column authored by a campaign surrogate and published in the Guardian newspaper that was titled "'Middle Class' Joe Biden has a corruption problem - it makes him a weak candidate."

Later that day, Biden released his ad accusing Sanders of being dishonest. When a CBS reporter asked Wednesday why Biden was still attacking Sanders despite accepting the apology, he drew a distinction between the two events.

"He apologized for saying that I was corrupt. He didn’t say anything about whether or not I was telling the truth about Social Security," Biden told reporters in Mason City.

Stephen Gruber-Miller covers the Iowa Statehouse and politics for the Register. He can be reached by email at sgrubermil@registermedia.com or by phone at 515-284-8169. Follow him on Twitter at @sgrubermiller.

Nick Coltrain is a politics and data reporter for the Register. Reach him at ncoltrain@registermedia.com or at 515-284-8361.

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