Washington (CNN) Sen. Bernie Sanders apologized to fellow 2020 contender Joe Biden for an op-ed by one of his campaign surrogates that suggested the former vice president has a "corruption problem."

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

The op-ed , written by Zephyr Teachout and published on Monday in The Guardian, argued that Biden's "record represents the transactional, grossly corrupt culture in Washington that long precedes Trump" and would give Trump the "perfect foil."

Teachout, an associate professor at Fordham Law who endorsed Sanders but is not a formal part of the senator's campaign, wrote the op-ed, but it was further spread by Sanders' campaign speechwriter, David Sirota, in his "Bern Notice" email newsletter.

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

Less than two weeks before the Iowa caucuses, the Sanders campaign has sharpened its attacks against Biden, with the latest criticism over Biden's record on Social Security.

Sanders told CBS News that he does not approve of his supporters' aggressive online attacks against the other 2020 Democratic candidates.

"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.