Mr. Sanders overlapped with Mr. Biden in the Senate for just two years, from January 2007, after his election, until Mr. Biden’s ascent to the vice presidency in 2009. Mr. Biden was not around all that much. Then a senator from Delaware and chairman of the Foreign Relations Committee, he spent much of 2007 prepping for his unsuccessful presidential campaign, the latter part of 2008 running with Mr. Obama and the rest of the time focusing on committee work.

“They basically had zero interaction, but I can’t recall Bernie ever saying a bad word about Biden, or vice versa,” said David Krone, chief of staff to Harry Reid, the former Democratic Senate majority leader from Nevada, who was close to Mr. Sanders.

The two men have sparred, but not savaged each other, in previous debates. Mr. Biden recently told a person in his orbit that he thinks Mr. Sanders, while lacking an understanding of foreign policy, “is basically a good guy.” One longtime Biden staff member summed up the former vice president’s view in Baby Boomer cultural terms: In the mid-1960s, when both men were in college, Mr. Biden was a square, striving, law-school-bound ex-jock who would have seen Mr. Sanders as a noisy, strident, scraggly but basically benign campus archetype — “the hippie.”

Mr. Sanders, an independent from Vermont who caucuses with the Democrats, has privately praised Mr. Biden as one of the few establishment senators to make him feel at home during his first few months in the Senate. (Mr. Obama, by contrast, later suggested in a memoir that Mr. Biden behaved a little arrogantly when he first arrived in 2005.)

But Mr. Sanders has not been shy about highlighting their policy differences over the years, especially his opposition to the Biden-backed bailout of financial firms during the 2008 financial crisis and Mr. Biden’s initial support for the war in Iraq.

In brief remarks after his Super Tuesday defeats, Mr. Sanders slammed Mr. Biden on Iraq and on his support for “disastrous trade agreements” that he said had cost millions of jobs. “You cannot beat Trump with the same old, same old kind of politics,” he added. In subsequent days, he attacked Mr. Biden’s record on abortion, same-sex marriage and the former “don’t ask, don’t tell” policy.

Mr. Sanders’s aggressive approach has, at times, angered Mr. Biden, who felt that Mr. Sanders had not acted quickly enough to muzzle supporters who posted racist and sexist attacks on two female officials of a Nevada union for opposing Mr. Sanders’s signature “Medicare for All” plan in February. “Disown them, flat disown them,” Mr. Biden told an interviewer at the time. “It’s outrageous.”