Bernie Sanders apologizes for negative Joe Biden op-ed penned by surrogate "I'm sorry that that op-ed appeared," Sanders said.

Vermont Sen. Bernie Sanders has apologized after a prominent surrogate published a negative op-ed about Sanders' 2020 rival former Vice President Joe Biden.

Author Zephyr Teachout penned the op-ed in the Guardian, which claimed "Joe Biden Has a Big Corruption Problem." The op-ed included sharp attacks of Biden's candidacy.

“Some think nominating Joe Biden, a moderate white man who calls himself “Middle Class” Joe, makes sense. But Biden has a big corruption problem and it makes him a weak candidate," wrote Teachout. "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.'"

The Sanders campaign recently promoted the op-ed in a newsletter called the "Bern Notice."

In an interview with CBS, Sanders apologized.

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

"[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," Sanders added.

“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,” Biden said in a tweet reacting reports of Sanders apology late Monday night.

The apology comes after the Sanders campaign has been criticized for sharpening attacks of their opponents. Sanders denied responsibility for volunteer talking points that attacked Sen. Elizabeth Warren's ability to expand the Democratic electorate. Biden also accused Sanders' campaign of sharing what he called a “doctored video” misleading voters about his stance on Social Security.

Sanders was also asked if he approves of his supporters who attack 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."

Instead, Sanders urged his supporters to communicate civilly.

"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."