Mr. Sanders, the senator from Vermont, and Mr. Biden, the former vice president, have resumed their attacks on each other, with back-and-forth sniping in the last week about Medicare for all and super PACs. Mr. Buttigieg and Amy Klobuchar have shown an increased willingness to go after Ms. Warren, the senator from Massachusetts.

Hours before the dinner was to begin, Former Representative Beto O’Rourke of Texas announced the end of his presidential campaign. His departure was so sudden that supporters had already planted scores of his signature black-and-white campaign signs outside the arena.

Lower-tier candidates looking to energize their campaigns have also been more pugnacious: Representative Tulsi Gabbard of Hawaii, for instance, has amped up her criticisms of the Democratic National Committee, the news media and even Hillary Clinton, who is not in the race.

But nowhere has the battle been more intense than at the event on Friday, with candidates trading barbs less on specific policies, and more on political ideology.

“We need big ideas, and here’s the critical part: We need to be willing to fight for them,” Ms. Warren said. “It’s easy to give up on a big idea, but when we give up on big ideas, we give up on the people whose lives would be touched by those ideas.”

Of the top-tier candidates, only Mr. Sanders — the sole candidate to stand stationary and speak from note on a lectern — did not attack his opponents, instead delivering a version of his standard stump speech that championed the working class and took aim at corporate interests and the rich. He quoted Franklin D. Roosevelt and Nelson Mandela and provided pleas for his signature policy proposals, including “Medicare for all” and tuition-free public college.