Conflict between Mr. Biden and Ms. Harris had the potential to define the evening, after their first clash in the June debate in Miami jolted both their candidacies and threatened to harm Mr. Biden’s. In that event, Ms. Harris delivered a searing rebuke of Mr. Biden’s political and legislative judgment, assailing the former vice president for having worked with segregationist senators in the 1970s to oppose school busing — a policy, she noted, that aided her own path through the public schools in Berkeley, Calif.
Mr. Biden was left grasping for a rebuttal and afterward indicated that he was personally stung by the attack; the contrast between Ms. Harris’s assured critique and his unsteady response dented Mr. Biden’s support in polls, where Democratic voters have tended to back him chiefly because they see him as a strong competitor for the general election against President Trump.
Yet Mr. Biden appears to have recovered much of his strength in polls over the intervening month. And while Ms. Harris’s support has risen, it is not clear whether she has managed to convert her debate-stage electricity into sustained gains: In many polls, she still trails not only Mr. Biden, but also Ms. Warren and Mr. Sanders.
But Mr. Biden and Ms. Harris were not alone onstage Wednesday, and the diverse array of competitors joining them was expected to present challenges for both, particularly for the former vice president. Perhaps most vexing was the presence of three liberals who are seeking a breakthrough moment, and who have not hesitated to draw sharp distinctions in the campaign: Mr. Booker, and Ms. Gillibrand and Mr. Castro, the former San Antonio mayor and federal housing secretary.
In his first debate in June, Mr. Castro startled the Democratic field with his uncharacteristically combative approach, harrying former Representative Beto O’Rourke of Texas with persistent attacks on his immigration proposals. There was ample material in the records of both Mr. Biden and Ms. Harris to invite the same approach.
And Ms. Gillibrand — who like Mr. Castro is struggling to qualify for the third round of debates in September — appeared to have provided a preview of an intended attack on Mr. Biden last weekend, alluding on the campaign trail to a candidate who had questioned the value of women working outside the home. Advisers to the former vice president took it as a reference to his opposition in the early 1980s to an expanded child tax credit; even before the debate got underway they circulated a memo to supporters predicting she and others onstage would attempt “to knock out Biden.”
But the most unpredictable figure may be Mr. Booker. Though not a brawler by nature, he has been increasingly eager to highlight differences between his record and Mr. Biden’s on criminal justice, recently calling him “the proud architect of a failed system.” Mr. Booker has been building a substantial field operation on the ground in the early primary states and methodically collecting endorsements from local power brokers there, but he has so far lacked a moment of ignition on the national level.