And Seth Masket, director of the Center on American Politics at the University of Denver, said Mr. Biden was “acting like a front-runner — sticking to his strengths, giving pretty concise answers, prioritizing — not making — mistakes over swinging for a home run.”

Klobuchar versus Buttigieg

One of the fiercest exchanges was between Senator Amy Klobuchar of Minnesota and Mayor Pete Buttigieg of South Bend, Ind. Ms. Klobuchar challenged Mr. Buttigieg on his experience and electability — noting that she has repeatedly won in conservative parts of Minnesota, while Mr. Buttigieg has never won a statewide election. Dan Pfeiffer, a former senior adviser to Mr. Obama, called the exchange “a significant moment in this primary.”

Strategists were divided, however, on who came out on top.

“The Amy-Pete clash went to Klobuchar,” Larry Sabato, director of the Center for Politics at the University of Virginia, tweeted. “Top-level experience & winning big elections matter.”

Mo Elleithee, executive director of the Georgetown Institute of Politics and a former spokesman for the Democratic National Committee, said the opposite. “Klobuchar continues to be one of the best debaters in the field, and she was solid tonight, but I think the experience attack on Pete was not a good moment,” he tweeted. “Pete continues to handle attacks better than almost anyone else in this format.”

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Sanders on race

Early in the debate, Senator Bernie Sanders was asked to comment on the lack of racial diversity onstage: Six of the seven candidates are white. Mr. Sanders pivoted back to the previous topic, climate change, and several commentators expressed frustration.

“While I’m happy that @SenSanders can tie race to environmentalism, the fact that he refused to answer the question about the lack of diversity at this #DemDebate shows he still hasn’t learned the lessons he should have in 2016,” tweeted Andra Gillespie, a political scientist at Emory University.

Mr. Sanders did, however, bring his answer back around to race, emphasizing the disproportionate effects climate change has on minority communities.