Warren Draws Fire From All Sides, Reflecting a Shift in Fortunes in Race

Ms. Warren was squeezed, at times, from the left as well: While Mr. Sanders never broke their informal nonaggression pact, he agreed with several of the moderates that it was “appropriate” to enumerate the financial trade-offs involved in single-payer health care, including taxes on Americans that would be “substantially less than what they were paying for premiums and out-of-pocket expenses.”

And while Mr. Sanders, who had a heart attack this month, was forced to address new concerns about his health, his campaign aides confirmed during the debate that he had secured an endorsement from Representative Alexandria Ocasio-Cortez of New York that could inject new energy into his candidacy.

But there were also the germs of a broader debate about the role of the United States in the Middle East: In an intense exchange between the two military veterans onstage, Representative Tulsi Gabbard of Hawaii said that it was not only Mr. Trump who had “the blood of the Kurds on his hands,” but also politicians in both parties and news media organizations that had cheered for “regime change war.”

Her remarks drew forceful pushback from Mr. Buttigieg, who said Ms. Gabbard was “dead wrong,” arguing that “the slaughter going on in Syria is not a consequence of American presence — it a consequence of a withdrawal and a betrayal by this president of American allies and American values.”

While Mr. Biden and Ms. Warren did not clash directly over foreign policy, they diverged in a stark fashion over the situation in Syria. Mr. Biden said he would want to keep American troops there and convey to the Turkish government that it would pay a “heavy price” for its invasion. Ms. Warren said she opposed Mr. Trump’s handling of the situation but believed the United States should “get out of the Middle East.”

Throughout the evening, Mr. Biden played a far less central role than he did in past debates, stepping to the foreground for exchanges over foreign policy but otherwise taking a more passive approach. His most important moment of the night may have come early on, when he was pressed by a moderator to explain why his son had not crossed any ethical lines by doing business in Ukraine while his father was overseeing diplomacy there for the Obama administration.

Mr. Biden said several times that he and his son had done “nothing wrong,” and alluded repeatedly to an interview Hunter Biden gave to ABC News, in which he said it had been an error in judgment to sit on the board of a Ukrainian gas company while the elder Mr. Biden was vice president. Mr. Trump has accused the Bidens of corruption, often in false or exaggerated terms, and his efforts to enlist the government of Ukraine in tarring Mr. Biden instigated an impeachment inquiry.