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“Bernie Sanders is out there making an appeal for his message and his policy ideas,” Faiz Shakir, Mr. Sanders’s campaign manager, said. “And they’re going to be the same regardless of whether it’s Joe Biden, Hillary Clinton or Barack Obama running against him.”

“It is not contingent upon the people in the race,” he said.

That kind of thinking could allow Mr. Sanders, of Vermont, to operate above the campaign fray and outside of the political gossip he hates. It also risks making him at times appear out of touch with the realities of the race: Mr. Sanders has steamrollered through successes and setbacks alike in single-minded pursuit. His ideological message has not deviated. He does not mention other candidates or allude to them. Even his irascible demeanor is unchanged.

Mr. Sanders’s campaign says he is taking his status as a leading candidate seriously, and is participating in decision-making behind the scenes — how to allocate money, where to travel, what message to emphasize and when. During campaign swings, he is making an effort to meet with local officials and union leaders. He has at times even talked about himself, something that he resisted in 2016 but that his advisers have urged him to do.

“I grew up in a family that lived paycheck to paycheck,” he said at a rally here in Pittsburgh, “and I know what that is about.” It was a start.

[Who’s in? Who’s out? Keep up with the 2020 field with our candidate tracker.]

Yet being at the top brings all kinds of perils, and candidates who start out as the highest-profile, best-funded contenders in the race have a mixed record.

Mrs. Clinton was a globally famous fund-raising powerhouse when she entered the presidential race in 2007, but Mr. Obama overtook her with an insurgent campaign that united wary voters. The same year, Rudolph W. Giuliani, the former New York mayor, saw an early polling lead in the Republican primary disintegrate after conservative voters discovered that behind his image as the hero after the Sept. 11 attacks was an eclectic political record that included liberal views on abortion and guns.