Thursday’s debate, the first since July, came at a moment when a race that once seemed volatile had become remarkably stable. Over the summer the field divided into two distinct classes, with Mr. Biden, Mr. Sanders and Ms. Warren representing the top tier in both national and early-state polling.
While Mr. Biden remains the front-runner in the Democratic race, he has yet to produce the kind of commanding debate performance that might excite undecided Democrats or put to rest their unease about his readiness for a contest with Mr. Trump. Mr. Biden’s support has held steady for the past few months, hovering around 30 percent of primary voters, but there is no indication he has won over any skeptical voters since entering the race in April.
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After a disastrous first debate in June, when Ms. Harris delivered a thumping denunciation of his record on race, Mr. Biden appeared somewhat more sure-footed during a July debate in Detroit.
The most significant change to the campaign over the course of the summer has been Ms. Warren’s steady rise — and the impact her surge has had on the trailing candidates. After months of training their fire at Mr. Biden, a few of the Democratic hopefuls have started to target the Massachusetts senator, specifically her array of ambitious policy proposals.
At the New Hampshire Democratic Convention last weekend, Mr. Booker and Mr. Buttigieg both invoked Ms. Warren’s much-discussed plans to make the case that ideas alone were insufficient to winning the presidency and enacting an agenda.
It was the first time either of the two candidates, who have needled Mr. Biden in ways that are both overt and subtle, had implicitly taken on Ms. Warren or reflected both her new status in the race and the urgency others are feeling to slow her rise.
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Mr. Sanders entered the debate under political pressure, too: With Ms. Warren gaining strength and his own poll numbers stalled, he cannot afford to let her emerge as the leading champion of their party’s populist left. Should a Biden-Warren rivalry come to dominate the next phase of the race, Mr. Sanders could find himself frozen in third place.