(Bloomberg) -- Kamala Harris renewed her criticism of Joe Biden for his opposition to busing as a senator in the 1970s, extending a dispute between the two Democratic presidential contenders sparked at last week’s debate.“I’ve asked him and have yet to hear him agree that busing, that was court-ordered and mandated in most places and in that era in which I was bused, was necessary and he has yet to agree that his position on this, which was to work with segregationist and oppose busing was wrong. Period,” the California senator told reporters Thursday in Indianola, Iowa.Biden, who also was campaigning in Iowa, said, “I don’t have to atone” and had always been a strong supporter of civil rights. He declined to criticize Harris, saying the Democratic presidential contenders must avoid engaging in “a circular firing squad” that will be damaging to all candidates.The spat is carried over from an exchange Harris and Biden had on stage during the first debate in Miami last week. Harris, one of two African American candidates vying for the black vote as a ticket to the White House, criticized him in deeply personal terms about his record of opposition, when he was in the Senate, to federally mandated busing to desegregate schools. While busing hasn’t been an issue in the Democratic campaign, race and equality are major points of discussion.“Do we need to do a quick lesson in history, which is that there were forces and individuals and supposedly leaders in our country who actively worked against the integration of schools based on race?” Harris, who’s seen her poll number jump since the debate, said Thursday. “But he has yet to agree that the position he took then was what’s wrong. And, and he and I just disagree on that. ”Local ControlBut she also faced questions about her own stance on whether busing should be federally mandated.“Thankfully we’re not in that position where we have governments standing in the way of integration of our schools, but we still do have segregation in our schools,” Harris said. “So it needs to be addressed and of course I am in support of whatever tool is available to those school districts to implement further integration.”Leaving it to school districts to determine whether to bus students was largely the argument Biden was making. The former vice president on Thursday said Harris was “absolutely right” that local education officials should determine whether busing should be part of a means to tackle segregation.Biden said he wouldn’t respond to attacks from rivals by digging up controversies from their pasts.“I’m not going to go back and use the same tactic they’re trying to use,” he said in Independence, where he walked in a Fourth of July parade. “I’m not going to go back and talk about the record of anyone from 10, 20, 30 years ago. There’s a lot out there that a lot of people would like to do differently than they did but everything is lost in context.”Asked if he understood that the conversation about race has changed, he said: “Absolutely, positively I do. As much or more than anyone.”On Wednesday, members of Biden’s communications team had dueled with Harris’s campaign on Twitter about the busing issue.Biden deputy campaign manager Kate Bedingfield tweeted in response that “it’s disappointing that Senator Harris chose to distort Vice President Biden’s position on busing — particularly now that she is tying herself in knots trying not to answer the very question she posed to him!”Harris’s national press secretary Ian Sams responded by saying that Biden had called busing an “asinine concept” and the back-and-forth continued with Sams later adding that he and Harris both thought Biden is “a good guy” and “That’s why a simple ‘working with segregationists to stop busing 40 years ago was wrong, and I shouldn’t have done it’ would be welcome.”He was referring to Biden’s comments from last month about working with two senators in the 1970s who were advocates of segregation.To contact the reporters on this story: Misyrlena Egkolfopoulou in Indianola, Iowa at megkolfopoul@bloomberg.net;Jennifer Epstein in Des Moines at jepstein32@bloomberg.netTo contact the editors responsible for this story: Joe Sobczyk at jsobczyk@bloomberg.net, Max BerleyFor more articles like this, please visit us at bloomberg.com©2019 Bloomberg L.P.