The debate also highlighted the considerable areas of agreement across the Democratic field on overarching policy goals, including taking aggressive action to counter climate change, expanding voting rights and restoring traditional American alliances around the world.
Ms. Warren used the impeachment inquiry, and the testimony on Wednesday by Gordon D. Sondland, the United States ambassador to the European Union and a Trump donor, to criticize the practice of installing wealthy political supporters in overseas embassies. Mr. Booker railed against Mr. Trump for what he described as his human rights violations at the southern border, such as “when children are thrown in cages.”
Ms. Harris jabbed that Mr. Trump “got punked” by Kim Jong-un, the North Korean dictator, in nuclear negotiations. For Ms. Klobuchar, it was Mr. Trump’s forgiving treatment of Saudi Arabia after its agents kidnapped and killed the journalist Jamal Khashoggi.
“That sent a signal to dictators around the world that that’s O.K.,” Ms. Klobuchar said.
Still, if there were no heated moments likely to endure past the evening, as there had been in past debates, there were fault lines within the field, separating the most progressive candidates, like Ms. Warren and Senator Bernie Sanders of Vermont, from comparatively moderate figures like Mr. Biden, Mr. Buttigieg and Ms. Klobuchar.
Mr. Buttigieg, who eagerly assailed Ms. Warren’s health care policies at the debate in October, was far more veiled in his criticism this time, arguing that Democrats must “galvanize not polarize” a coalition representing a majority of voters. And Ms. Klobuchar suggested that there were candidates who were making big promises “because they sound good on a bumper sticker and then throw in a free car.” But neither of them named names or tried a direct attack as they had done last month. Similarly, Ms. Harris, whose campaign has been on a downward spiral in the polls, notably declined an opportunity to confront Mr. Buttigieg.
The leading candidates of the left took much the same approach. Mr. Sanders, defending his support for single-payer health care, referred to skeptical competitors who he said believed “that we should not take on the insurance company, we should not take on the pharmaceutical industry.” But he did not say to whom he was referring.