Sen. Bernie Sanders apologized to Joe Biden late Monday for an op-ed a prominent supporter wrote accusing the former vice president of having a “big corruption problem,” and Sanders urged his backers to exercise civility on the heels of heated disputes with his rivals.

In an interview with CBS News, Sanders (I-Vt.) expressed contrition for the op-ed, which was written by law professor Zephyr Teachout for the Guardian newspaper and was promoted by the Sanders campaign in a newsletter. Teachout endorsed Sanders last year, and his campaign touted her support.

“It is absolutely not my view that Joe is corrupt in any way. And I’m sorry that that op-ed appeared,” Sanders said in the interview. A Sanders campaign spokesman confirmed his comments.

Biden replied to Sanders on Twitter. “Thanks for acknowledging this, Bernie. These kinds of attacks have no place in this primary. Let’s all keep our focus on making Donald Trump a one-term president,” he wrote.

It is rare for candidates to apologize for their actions or the activities of their supporters, particularly with the Iowa caucuses happening so soon, on Feb. 3.

The remarks marked at least a temporary de-escalation between two rivals who sit at or near the top of the polls and who had grown increasingly hostile to each other in recent days. Sanders also appeared keen on calming tensions with Sen. Elizabeth Warren (D-Mass.); the two walked arm-in-arm at a Martin Luther King Jr. Day event in South Carolina six days after an explosive post-debate confrontation centering on a private conversation about gender and elections.

It was also a shift in tone for the Democratic race, which just 24 hours earlier had reached new levels of contentiousness. With President Trump’s impeachment trial set to begin in earnest in the Senate this week, some Democrats have been hoping the party will project more unity.

Some Sanders supporters have aggressively criticized Biden and Warren on social media. Sanders told CBS he did not approve of personal attacks. “If anyone knows me, what I believe is we need a serious debate in this country on issues. We don’t need to demonize people who may disagree with us,” he said.

Sanders added, “I appeal to my supporters: Please, engage in civil discourse.” Sanders said his supporters were not the only ones guilty of crossing the line. “We’re not the only campaign that does it. Other people act that way as well. I would appeal to everybody: Have a debate on the issues. We can disagree with each other without being disagreeable, without being hateful. That is not what American politics should be about.”

In her op-ed, Teachout wrote that “Biden has a big corruption problem and it makes him a weak candidate.” She added, “I know it seems crazy, but a lot of the voters we need — independents and people who might stay home — will look at Biden and Trump and say: ‘They’re all dirty.’ ”

She accused Biden of being too close to the credit card, health-care and energy industries. Sanders adviser David Sirota highlighted the op-ed in his “Bern Notice” newsletter, which is distributed electronically.

Some Sanders campaign officials have long been pushing for the senator to be more aggressive against Biden. They have achieved mixed results. As recently as Sunday, Sanders directly attacked Biden’s record on Social Security.

“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 in response to a question from The Washington Post as he left a radio interview in New Hampshire. “I don’t think that that is disputable.”

But Sanders has also shied away from other opportunities to go after at Biden, including at a debate last summer. On Sunday, he called Biden a “nice guy” and a “decent person,” chalking up Biden’s strong standing among older voters to those traits.

At a Brown & Black Forum in Iowa on Monday, Biden said that he would not pursue any changes to Social Security that Sanders had referred to. “There will be no compromise on Medicare and Social Security, period,” Biden said. “That’s a promise.”

On Saturday, Biden criticized a video of him speaking about the topic that a Sanders aide publicized online, asserting without evidence that it was “doctored.” That did not appear to be the case. But fact-checkers said the video was trimmed in a way that was misleading.

Sanders acknowledged Sunday that the video should have had “the whole context,” even as he defended his larger critique, citing Biden’s past support for a balanced-budget amendment as evidence the former vice president has been hostile to Social Security.

Speaking to reporters after Sanders’s remarks at the Monday forum, Nina Turner, a national co-chair of the Sanders campaign, said that the senator and Biden were friends and he would not insult him.

“They disagree on many issues,” she said, “but you have never heard Senator Bernard Sanders call Vice President Biden corrupt.”

David Weigel in Des Moines contributed to this report.