The hedge fund billionaire Tom Steyer — who has previously been an ally of the Vermont senator — joined in and so did Senator Elizabeth Warren, who contrasted herself with Mr. Sanders for the first time by saying she would be more effective as president.

In response to the onslaught, Mr. Sanders joked of his newfound front-runner status. “I’m hearing my name mentioned a little bit tonight,” he said. “I wonder why.”

Time and again Mr. Sanders was challenged on the political impact of his expansive agenda — and how he would pay for it. “Can you do the math for the rest of us?” asked Norah O’Donnell of CBS. Mr. Sanders declined.

“How many hours do you have?” he said, without providing details.

It was a messy two-hour affair, marked by moderators who went missing for long stretches as cross-talk, over and over, made the conversation unintelligible.

“Why am I stopping? No one else stops,” Mr. Biden wondered aloud.

When the moderators did intervene, it was sometimes to move the conversation past key moments, including when Mr. Biden pointedly suggested some complicity for Mr. Sanders in a mass shooting because of past positions on gun legislation, and when Ms. Warren began swiping at Mr. Sanders.