That was Biden, building toward something big only to peter out.

I’m focusing so much on him because so much of the focus was on him. The debate in Detroit among the second group of 10 Democrats wasn’t principally about health care, immigration, race relations or climate change — not in terms of the question foremost on many observers’ minds, the matter that cried out loudest for resolution. The debate was about whether the man at the prime lectern, with a very big lead, had lost his famous ebullience and parted with his former confidence.

He was quarry: That much was indisputable. Someone who’d never read a word about the contest for the Democratic presidential nomination before Wednesday night could have tuned into almost any 10-minute stretch and figured out that he was the front-runner, because the overwhelming majority of arrows were flying in his direction (though some very sharp ones came Harris’s way).

De Blasio was perhaps the most insistent archer. The most obnoxious, too. And it was powerful testimony to Biden’s enduring political skill that he turned that into a joke. “I love your affection for me,” Biden told de Blasio. “You spend a lot of time with me.”

Biden’s ability to keep from sweating and to keep smiling was no mean feat under the circumstances. He parried Gillibrand by noting that in years past, she was a fan of his. “I don’t know what’s happened except that you’re now running for president,” he said.

But in his back-and-forth with Booker, during which he disparaged Booker’s approach to policing as mayor of Newark, he lacked Booker’s crispness and clearness. And Booker got the better of him. “Mr. Vice President,” he said, “there’s a saying in my community — you’re dipping into the Kool-Aid and you don’t even know the flavor.”

A few facets of Biden’s candidacy came into sharper relief. One was how much he’s riding — and counting — on his association with President Barack Obama and how complicated that tactic is. Biden summoned him as a character witness, but that opened the door for rivals to press Biden on parts of Obama’s presidency — all the deportations of undocumented immigrants, for example — that don’t look so rosy or fit so neatly into Democratic priorities now.

And it gave Booker the grist to portray Biden as a shifty opportunist. “You can’t have it both ways,” Booker said. “You invoke President Obama more than anybody in this campaign. You can’t do it when it’s convenient and then dodge it when it’s not.”