Senator Elizabeth Warren and former Vice President Joseph R. Biden Jr. were well behind, and the political outsider Andrew Yang announced that he was ending his campaign. The next contest is the Nevada caucuses on Feb. 22.

Takeaway: The Democratic contest seems to coming down to the question of whether the party will move to the left with Mr. Sanders, a democratic socialist, or to the center with Mr. Buttigieg or even Senator Amy Klobuchar, who was in a strong third.

Another angle: Michael Bloomberg, set to join the ballot in later states, was on the defensive after he was heard on resurfaced audio defending how minorities were targeted in a now-discontinued New York police policy of stop-and-frisk.