Des Moines, Iowa — Senator Bernie Sanders apologized to former Vice President Joe Biden on Monday for an op-ed written by one of his campaign surrogates that claimed Biden has a "big corruption problem."

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

The op-ed, published in The Guardian by law professor Zephyr Teachout, claims Biden "has perfected the art of taking big contributions, then representing his corporate donors at the cost of middle- and working-class Americans."

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"Converting campaign contributions into legislative favors and policy positions isn't being 'moderate,'" Teachout wrote. "It is the kind of transactional politics Americans have come to loathe."

While Teachout does not officially work on the Sanders campaign, she stumps for him, and has introduced and endorsed him.

Her op-ed comes at a tense moment between Sanders and Biden. Over the last week, Sanders and his team have critiqued Biden's past remarks on Social Security funding, using a video that takes Biden's comments out of context and appears to show him agreeing with former Republican Speaker of the House Paul Ryan about privatizing the program.

Biden, in response, accused the Sanders campaign of releasing "doctored" footage, and asked Sanders to apologize.

Sunday before a New Hampshire rally, Sanders told CBS News: "[Joe] is a decent person. He is a friend of mine. People like him. And we're not going to make personal attacks on Joe Biden but I think the record shows that Joe's history in the Senate and my history in Congress are very different."

"I think it is important to know where a senator has come from and where his head is at. And Joe's record on [Social Security] is a little different than mine," said Sanders.

Biden responded Monday by telling VICE News that the Sanders campaign "lied" about his record.

Sanders has also recently been fighting with rival Elizabeth Warren, after she alleged that Sanders told her a woman couldn't win the presidency. Sanders supporters, who have a fervent Twitter presence, helped to make #RefundWarren and #BidenSocialSecurityCuts some of the top trending topics in the country this past week.

CBS News asked Sanders if he approved of his supporters aggressively attacking his opponents online.

"No, I really don't," he said. "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."

"I appeal to my supporters: Please, engage in civil discourse," he added. "And by the way, 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."

Later Monday night, Biden night thanked Bernie for his remarks via Twitter.

"These kinds of attacks have no place in this primary," the former vice president added. "Let's all keep our focus on making Donald Trump a one-term president."