Joe Biden on Wednesday lashed out at his Democratic rivals who had condemned his fond recollections of working relationships with segregationists in the Senate, declining to apologize and defending his record on civil rights.

The angry exchange shattered, at least for now, the relative comity that had marked the Democratic presidential primary. A day after Biden invoked the 1970s, an era when he said he could find common ground with other senators — even virulent segregationists — his opponents offered their sharpest criticism yet.

Sen. Kamala Harris of California said the former vice president “doesn’t understand the history of our country and the dark history of our country,” and Sen. Cory Booker of New Jersey said Biden should immediately apologize for using segregationists to make a point about civility in the Senate.

Harris and Booker, who are both black, were not alone: Other candidates including Sens. Elizabeth Warren of Massachusetts and Bernie Sanders of Vermont also weighed in with criticism.

Yet for much of the day, Biden and his campaign appeared intent on defending, or at least explaining, his worldview of politics — which is rooted in his early days in the Senate when, he said, legislators who disagreed still worked together. He cited two defenders of segregation, Sens. James O. Eastland of Mississippi and Herman E. Talmadge of Georgia, to make that point.

Later Wednesday evening, Biden was sharper in his criticism of the two former Southern senators. “We in fact detested what they stood for in terms of segregation and all the rest,” he said.

Biden, a longtime supporter of the Voting Rights Act who has cited the civil rights movement as motivation for getting into politics, has many African-American allies, and on Wednesday a number of prominent black leaders defended him, including Rep. James E. Clyburn, D-S.C., the highest-ranking African-American in Congress.

But for many Democrats, Biden’s decision to highlight those relationships seemed deeply misguided and out of touch with political change in America.

“I just really don’t understand for the life of me what the vice president could have been thinking,” said Leah Daughtry, a veteran Democratic strategist who is African-American.

This article originally appeared in The New York Times.