Kamala Harris Makes the Case That Joe Biden Should Pass That Torch to Her

This was the moment Joseph R. Biden Jr. had to know would be coming. He did not seem entirely prepared.

About an hour into the debate on Thursday, Senator Kamala Harris, a black former prosecutor from California, had edged into the cross-talk with a request: “I would like to speak on the issue of race.”

Soon, her precise intention was clear. She paused and took a breath. “I’m going to now direct this at Vice President Biden,” she said, turning toward the 2020 primary front-runner who waxed nostalgic earlier this month about working with segregationist senators.

“I do not believe you are a racist,” she began. She had much more to say.

With that remark, Ms. Harris brought to the fore the central questions threatening Mr. Biden’s tenuous perch atop the polls.

Will the Democratic Party of 2020 — buoyed last year by the midterm success of a new generation of female and nonwhite candidates — really choose as its standard-bearer a center-left white man who joined the Senate during the Nixon administration? And if not, who will be the contender willing to challenge him most directly, with so many voters chiefly concerned about uniting to defeat President Trump?

As Ms. Harris spoke, Mr. Biden’s head angled down, cocking to the left. He blinked repeatedly. He clutched both ends of the lectern like a flotation device.

Entering Thursday evening, Mr. Biden had subsisted as a peculiar kind of favorite: 76 and rusty, his speeches heavier on curious digression than stirring crescendo, unapologetic about his affection for a bygone era of comity. And that, to Ms. Harris, was precisely the problem.