At the moment, no one is close to being the last candidate standing. But unless another Democrat rapidly consolidates support, Mr. Sanders could continue to win primaries and caucuses without broadening his political appeal, purely on the strength of his rock-solid base on the left — a prospect that alarms Democratic Party leaders who view Mr. Sanders and his slogan of democratic socialism as wildly risky bets in a general election.

The Biden team stoked that sense of alarm on Wednesday: Representative Cedric Richmond of Louisiana, a co-chairman of Mr. Biden’s national campaign and a former chairman of the Congressional Black Caucus, warned on a conference call with reporters that Democrats would risk “down-ballot carnage” if they selected Mr. Sanders.

“If Bernie Sanders was at the top of the ticket, we would be in jeopardy of losing the House,” Mr. Richmond said. “We would not get the Senate back.”

Yet in a reflection of the multidimensional melee that allowed Mr. Sanders to claim victory in New Hampshire with the smallest plurality of any winner in decades, Mr. Richmond also criticized two other candidates, Mr. Bloomberg and Mr. Buttigieg, lumping them into the same risky group and arguing that Democrats should not “take a chance with a self-defined socialist, a mayor of a very small city, a billionaire who all of a sudden is a Democrat.”

Mr. Mellman said Mr. Sanders would continue to benefit as long as there was a relative abundance of moderate candidates in the race. “The longer more of those people stay in,” he said, “the easier it is for Sanders to skate through.”

There is no sign that any of the half-dozen major candidates left in the race are headed for the exits: Mr. Buttigieg and Mr. Biden will have to contend in the Nevada caucuses against Senator Amy Klobuchar of Minnesota, who finished a strong third in New Hampshire, while on the left Mr. Sanders still faces a dogged competitor in Ms. Warren. Unless one candidate comes out of Nevada and South Carolina with a powerful upper hand, it is quite likely that the same atomized delegate count could continue into Super Tuesday, when 15 states and territories, accounting for nearly 40 percent of all delegates in the Democratic race, cast ballots on March 3.