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CHICAGO — Sen. Bernie Sanders apologized Monday night for an op-ed written by a campaign supporter that argued former Vice President Joe Biden “has a big corruption problem.”

In an interview in Iowa with CBS News, Sanders said “it is absolutely not my view that Joe is corrupt in any way. And I'm sorry that that op-ed appeared."

Biden quickly accepted Sanders' apology, writing in a tweet, "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."

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. https://t.co/tmFbLm98xd — Joe Biden (Text Join to 30330) (@JoeBiden) January 21, 2020

Sanders' vocal speechwriter David Sirota shared the op-ed, written by associate Fordham University law professor Zephyr Teachout, to an email list of supporters Monday morning. The email highlighted that “In one passage, she asks ‘When he pushed for cuts to Social Security, was he (Biden) serving donors or his constituents?’”

The op-ed was the latest in an ongoing clash between Sanders and the former vice president over Social Security, but was another example of Sanders’ supporters and staff taking a more aggressive tone than the candidate himself.

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Sanders spent the weekend, in conversations with reporters, hitting his top rival for previously supporting cuts to Social Security, while also stressing that Biden “is a decent person. He's a friend of mine, and people like him.”

“And we're not going to make personal attacks on Joe Biden,” Sanders said Sunday afternoon.

In 2016, Sanders was criticized for hurting the party with a protracted primary contest against Hillary Clinton. This time around, Sanders calls several of his rivals “friends.”

He was the first candidate to sign a #WeAreIndivisible pledge last spring that promised, in part, to “immediately” endorse the Democratic nominee. He regularly makes clear his confidence of the other Democratic candidates.

However, for the past two weeks, and with different candidates, Sanders and his campaign have been at the center of the field’s biggest clashes. These clashes were started by or escalated with the words or actions of Sanders campaign staff and surrogates, prompting responses by the candidate.

“As Democrats I thought we all knew this election was too important to attack other Democrats,” Biden wrote in a Monday fundraising email to supporters after the two went back and forth over the weekend.

“But Bernie Sanders and his campaign don’t care about that,” Biden’s email to supporters continued, “They’ve decided to unleash a barrage of negative attacks lying about and distorting my record.”

Saturday, Biden demanded an apology from the Sanders campaign, after it shared a series of video clips highlighting Biden’s openness to cuts or freezes to Social Security. One clip retweeted by an adviser was misleading and out of context, and suggested Biden praised cuts to Social Security proposed by former House Speaker Paul Ryan.

The previous week, Sanders battled with Sen. Elizabeth Warren, D-Mass., in another clash that began as a result of the actions of some in his campaign, who created volunteer talking points attacking his progressive ally about recollections of a private 2018 conversation.

The growing disputes come with the Iowa caucuses less than two weeks away. In the days before last week’s debate, top surrogate and campaign co-chair Nina Turner wrote an op-ed that criticized Biden's past policies affecting black America. Senior adviser Jeff Weaver also released a strong statement, challenging Biden's Iraq War vote.

Now, Sanders is urging his campaign to focus.

Sanders told CBS News he does not approve of the aggressive online attacks by some of his supporters. While noting that it is not just a problem with his campaign, he said “I appeal to my supporters: Please, engage in civil discourse.”