Mr. Biden, he said, “wants to take credit for Obama’s work, but not have to answer to any questions.”
Mr. Biden responded as though Mr. Castro’s complaint was that Mr. Biden was insufficiently defensive of the former president.
“I stand with Barack Obama all eight years,” Mr. Biden said. “Good, bad and indifferent.”
Mr. Biden’s fealty to Mr. Obama, throughout the debate, was consistent with his overall approach to the campaign. He has staked out a position, unapologetically, as a candidate of the Democratic center, building a sizable but far from dominant base of support, anchored in the admiration of moderates, older voters and African-Americans. But he has yet to expand his appeal beyond that base, which appears to make up between a quarter and a third of the Democratic electorate.
His resilience, though, has prompted some of his rivals to recalibrate their approach as the race enters the fall. After unleashing one of the contest’s toughest attacks against Mr. Biden in the first debate, Senator Kamala Harris of California steadfastly avoided critiquing the former vice-president or any of her Democratic opponents.
Ms. Harris used her opening statement to speak directly to, and criticize, President Trump. During the health care contretemps she lamented that “not once have we talked about Donald Trump.” And when she made the case for turning to executive action to overcome legislative gridlock, she turned to Mr. Biden, let out a laugh and borrowed Mr. Obama’s signature line. “Hey Joe, let’s say ‘yes we can,’” she said.
Ms. Harris’s attempt at a strategic makeover was hard to miss, but other candidates also tried to show voters a fuller version of themselves. Ms. Warren, while staying true to her vision for sweeping policy proposals, repeatedly sought to use a debate that was held in the city where she went to college to talk more about a personal story that many voters are dimly aware of. She recalled her Oklahoma youth, repeatedly cited her brothers’ military service and talked about being a public-school teacher.
If Ms. Warren seemed determined to unfurl her biography, Mr. Sanders came prepared to take on Mr. Biden. Mr. Sanders was assertive about drawing contrasts between his progressive credentials and Mr. Biden’s far more varied record. Where Mr. Sanders shied away from direct conflict the last time he shared a debate stage with Mr. Biden, in June, this time he sought out areas of sharp disagreement, including over the NAFTA trade deal and the war in Iraq. Mr. Biden supported both; Mr. Sanders opposed them.