Bernie Sanders apologised to Joe Biden on 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, Sander 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 apologise for their actions or the activities of their supporters, particularly with the Iowa caucuses happening so soon, on February 3.

The Democrat challengers to Trump in 2020 Show all 25 1 /25 The Democrat challengers to Trump in 2020 The Democrat challengers to Trump in 2020 Bernie Sanders The Vermont senator has launched a second bid for president after losing out to Hilary Clinton in the 2016 Democratic primaries. He is running on a similar platform of democratic socialist reform Getty The Democrat challengers to Trump in 2020 Joe Biden The former vice president recently faced scrutiny for inappropriate touching of women, but was thought to deal with the criticism well and has since maintained a front runner status in national polling EPA The Democrat challengers to Trump in 2020 Elizabeth Warren The Massachusetts senator is a progressive Democrat, and a major supporter of regulating Wall Street Reuters The Democrat challengers to Trump in 2020 Amy Klobuchar Klobuchar is a Minnesota senator who earned praise for her contribution to the Brett Kavanaugh hearings Getty The Democrat challengers to Trump in 2020 Michael Bloomberg Michael Bloomberg, a late addition to the 2020 race, announced his candidacy after months of speculation in November. He has launched a massive ad-buying campaign and issued an apology for the controversial "stop and frisk" programme that adversely impacted minority communities in New York City when he was mayor Getty Images The Democrat challengers to Trump in 2020 Tulsi Gabbard The Hawaii congresswoman announced her candidacy in January, but has faced tough questions on her past comments on LGBT+ rights and her stance on Syrian President Bashar al-Assad Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Pete Buttigieg The centrist Indiana mayor and war veteran would be the first openly LGBT+ president in American history Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Deval Patrick The former Massachusetts governor launched a late 2020 candidacy and received very little reception. With just a few short months until the first voters flock to the polls, the former governor is running as a centrist and believes he can unite the party's various voting blocs AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Beto O'Rourke The former Texas congressman formally launched his bid for the presidency in March. He ran on a progressive platform, stating that the US is driven by "gross differences in opportunity and outcome" AP The Democrat challengers to Trump in 2020 DROPPED OUT: Kamala Harris The former California attorney general was introduced to the national stage during Jeff Sessions’ testimony. She has endorsed Medicare-for-all and proposed a major tax-credit for the middle class AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Bill De Blasio The New York mayor announced his bid on 16 May 2019. He emerged in 2013 as a leading voice in the left wing of his party but struggled to build a national profile and has suffered a number of political setbacks in his time as mayor AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Steve Bullock The Montana governor announced his bid on 14 May. He stated "We need to defeat Donald Trump in 2020 and defeat the corrupt system that lets campaign money drown out the people's voice, so we can finally make good on the promise of a fair shot for everyone." He also highlighted the fact that he won the governor's seat in a red [Republican] state Reuters The Democrat challengers to Trump in 2020 DROPPED OUT: Cory Booker The New Jersey Senator has focused on restoring kindness and civility in American politics throughout his campaign, though he has failed to secure the same level of support and fundraising as several other senators running for the White House in 2020 Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Wayne Messam Mayor of the city of Miramar in the Miami metropolitan area, Wayne Messam said he intended to run on a progressive platform against the "broken" federal government. He favours gun regulations and was a signatory to a letter from some 400 mayors condemning President Trump's withdrawal from the Paris Climate Accord Vice News The Democrat challengers to Trump in 2020 DROPPED OUT: Kirsten Gillibrand The New York Senator formally announced her presidential bid in January, saying that “healthcare should be a right, not a privilege” Getty The Democrat challengers to Trump in 2020 DROPPED OUT: John Delaney The Maryland congressman was the first to launch his bid for presidency, making the announcement in 2017 AP The Democrat challengers to Trump in 2020 DROPPED OUT: Andrew Yang The entrepreneur announced his presidential candidacy by pledging that he would introduce a universal basic income of $1,000 a month to every American over the age of 18 Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Julian Castro The former San Antonio mayor announced his candidacy in January and said that his running has a “special meaning” for the Latino community in the US Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Marianne Williamson The author and spiritual adviser has announced her intention to run for president. She had previously run for congress as an independent in 2014 but was unsuccessful Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Eric Swalwell One of the younger candidates, Swalwell has served on multiple committees in the House of Representatives. He intended to make gun control central to his campaign but dropped out after his team said it was clear there was no path to victory Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Seth Moulton A Massachusetts congressman, Moulton is a former US soldier who is best known for trying to stop Nancy Pelosi from becoming speaker of the house. He dropped out of the race after not polling well in key states Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Jay Inslee Inslee has been governor of Washington since 2013. His bid was centred around climate change AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: John Hickenlooper The former governor of Colorado aimed to sell himself as an effective leader who was open to compromise, but failed to make a splash on the national stage Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Tim Ryan Ohio representative Tim Ryan ran on a campaign that hinged on his working class roots, though his messaging did not appear to resonate with voters Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Tom Steyer Democratic presidential hopeful billionaire and philanthropist Tom Steyer is a longtime Democratic donor AFP/Getty

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 Elizabeth Warren.; 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 centring 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 criticised 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 demonise 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 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 criticised a video of him speaking about the topic that a Sanders aide publicised 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 on 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' 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."