It was that kind of evening. The Democratic candidates vying for their party’s presidential nomination agreed that their main goal is to defeat Mr. Trump in the general election next November. But to win the nomination, they have to find ways of differentiating themselves from one another without making it look as if they are engaging in an unedifying exercise in the narcissism of small differences.

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If any particular candidate was on the menu on Thursday, it was Mr. Buttigieg, who is leading in the Iowa polls and drew some sharp words from Ms. Klobuchar. After he said that his experience on immigration came from real life and not “committees,” she said he had “basically mocked the hundred years of experience on the stage.”

They went back and forth for a while, trading mean remarks about who had experience in what. The exchange ended badly for him when she mentioned that although it is true that he was elected mayor of South Bend, Ind., he had lost a statewide race “by 20 points.”

It’s not clear whether Democratic voters unsure of whom to support learned anything new about the candidates at the debate, the sixth since June. (There are at least four more to come in the next two months.)

Ms. Klobuchar mentioned that her grandfather, an iron-ore miner, saved his money in a coffee can. Ms. Warren said that her three brothers had political differences but were united in their resentment of Amazon’s sophisticated tax-avoidance techniques.

Former Vice President Joe Biden looked tan and relaxed and did not do that thing where, when he begins to tell a long, meandering anecdote, you wonder at what point in the day he will arrive at his final verb and wrap up the sentence. He refused to say whether or not he would run for a second term, if he gets elected president in the first place.

Andrew Yang proved once again that he has a good sense of humor about himself — “I know what you’re thinking, America: How am I still on this stage with them?”— and declared, apropos of not much, that “if you get too many men alone and leave us alone for a while, we kind of become morons.”