While Democratic Party Leaders Fretted, Bernie Sanders Rose to Top

“If we go through March 3 and don’t show that we have a nominee who can appeal to a broad swath of our party, we’re going to be in serious trouble,” said Terry McAuliffe, the former Virginia governor and Democratic National Committee chairman, who endorsed Mr. Biden over the weekend.

Even now, authority figures like House Speaker Nancy Pelosi, Senate Minority Leader Chuck Schumer and former President Barack Obama have stayed on the sidelines, convinced their intervention would only fuel Mr. Sanders’s claims of an insider plot against him. Influential labor groups and advocacy organizations have either stayed quiet or supported multiple candidates.

The candidates themselves have made fitful or flawed efforts to garner mainstream Democratic support: A number of lawmakers open to endorsing Mr. Biden said he did little to reach out to them for most of the campaign, letting weeks or months go by without any contact from him or his aides.

A flashy late entrant, Mr. Bloomberg, the billionaire former New York City mayor, initially impressed elected officials with an energetic charm offensive and a war-machine campaign operation. But many Democrats developed serious reservations about him after a damaging debate-stage clash with Ms. Warren last month.

Underdog moderates persisted in pleading with party leaders for help, promising to continue their campaigns even at the risk of playing a spoiler’s role for Mr. Biden or Mr. Bloomberg. Last week, only three days before he dropped out of the race, Mr. Buttigieg visited the Capitol to seek endorsements and vowed in a private meeting with centrist Democrats that he would not withdraw. It was only after South Carolina, when internal campaign projections were clear that they faced a rout on Super Tuesday, that Mr. Buttigieg and Ms. Klobuchar dropped out.

And while Mr. Sanders has made private overtures toward centrists, his harpoons toward “corporate Democrats” and the scorched-earth tactics of his supporters have continued to unnerve Democratic leaders.

Christine Pelosi, one of the speaker’s daughters, last month contacted Mr. Sanders’s advisers to convey her unhappiness that the actress Susan Sarandon, who is backing the Vermont senator, had called for Speaker Pelosi’s ouster, according to a Democrat familiar with the conversations.