“Whether or not Biden is making choices to please donors, there is no doubt his record represents the transactional, grossly corrupt culture in Washington that long precedes Trump,” wrote Zephyr Teachout, a progressive law professor.

The op-ed, published in The Guardian, was pushed out by the Sanders campaign newsletter, “Bern Notice.”

Sanders told CBS News: “It is absolutely not my view that Joe Biden is corrupt in any way. And I’m sorry that that op-ed appeared.”

Biden later thanked Sanders 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.”

Though the candidates exchanged niceties, supporters on both sides showed no signs of a truce, calling on each other to retract negative statements about the other.

The heightened tensions come just two weeks before the first 2020 contest gets underway in Iowa.

A new Iowa poll released Monday, which had Biden leading a tightly packed field, gave some indication that recent intraparty fighting turned off potential caucus-goers. The poll was in the field in the days following last week’s Democratic national debate, where Warren and Sanders engaged in a brief post-debate confrontation related to Warren’s assertion that Sanders privately told her he didn’t think a woman could win the presidency, something Sanders has denied.

When those polled were asked if they would not support a candidate based on the debate, only two candidates ranked in the double digits: Warren at 12 percent and Sanders at 11 percent.