The race is heating up between Joe Biden (D) and Sen. Bernie Sanders (I-VT), who are in the midst of a contentious back and forth following the former vice president’s negative ad, accusing Sanders of launching “dishonest attacks.”

“As Democrats, we can’t launch dishonest attacks against fellow Democrats. We have to beat Donald Trump. Now Bernie’s campaign has unleashed a barrage of negative attacks on Joe Biden,” the ad’s narrator states.

“They even accused Joe Biden of supporting Paul Ryan’s cuts to Social Security. Bernie’s campaign is not telling the truth,” it continues, claiming Biden has “repeatedly voted to save Social Security.”

“Bernie’s negative attacks won’t change the truth,” it concludes. “Joe Biden is still the strongest Democrat to beat Donald Trump”:

I've been fighting to protect — and expand — Social Security for my whole career. Any suggestion otherwise is just flat-out wrong. pic.twitter.com/KWIIJgFqGk — Joe Biden (Text Join to 30330) (@JoeBiden) January 22, 2020

Sanders’ campaign manager Faiz Shakir responded by accusing Biden of, “trying to distort his decades-long record of proposing and voting for cuts to Social Security benefits for millions of people.”

He said:

Joe Biden just released the first negative ad of the 2020 Democratic primary, and let’s be clear about why: he’s trying to distort his decades-long record of proposing and voting for cuts to Social Security benefits for millions of people. Joe Biden is no defender of Social Security, and a negative ad won’t help him outrun his record.

Sanders also responded to Biden’s ad on social media.

“Let’s be honest, Joe. One of us fought for decades to cut Social Security, and one of us didn’t. But don’t take it from me. Take it from you,” Sanders tweeted alongside a video of Biden in his own words:

Let’s be honest, Joe. One of us fought for decades to cut Social Security, and one of us didn’t. But don’t take it from me. Take it from you. pic.twitter.com/qh7qb1Hmcl — Bernie Sanders (@BernieSanders) January 22, 2020

Their bitter battle over Social Security is not new. Biden recently accused the Sanders campaign of spreading a “doctored video,” showing the former vice president seemingly on board with cutting Social Security:

The video in question, circulated on Twitter by a top Sanders adviser, does not appear to be altered. But the short clip omits Biden’s larger argument over how Ryan handled the 2017 tax cuts and subsequent budget debates. A separate Sanders’ adviser included a transcript of Biden’s remarks in the video clip in a separate campaign newsletter. He added other, more extended video, of Biden as a U.S. senator in 1995 and presidential candidate in 2007 explaining his support for a more austere federal budget, including putting Social Security and Medicare “on the table.”

The recent attacks follow Sanders’ apology for an op-ed penned by surrogate Zephyr Teachout, who wrote that the former vice president has “a big corruption problem.”

“It is absolutely not my view that Joe is corrupt in any way,” Sanders told CBS News. “And I’m sorry that that op-ed appeared.”

This is not the first time Sanders has attacked Biden’s record. He went after his opponent just hours before the Democrat debate in Iowa last week, tweeting a video featuring David Sirota, his speechwriter, “fact-checking” Sanders’ claims about Biden:

When you look at my record vs. Joe Biden's record, I just don't think that Biden's record is going to bring forth the energy that we need to defeat Trump. pic.twitter.com/Y8UJYJT6wT — Bernie Sanders (@BernieSanders) January 14, 2020

A CNN poll released Wednesday shows Sanders leading the Democrat field, with 27 percent to Biden’s 24 percent.