Get live updates about the Florida primary.

_____

The scenario Karl Rove outlined was bleak.

Addressing a luncheon of Republican governors and donors in Washington on Feb. 19, he warned that Donald J. Trump’s increasingly likely nomination would be catastrophic, dooming the party in November. But Mr. Rove, the master strategist of George W. Bush’s campaigns, insisted it was not too late for them to stop Mr. Trump, according to three people present.

At a meeting of Republican governors the next morning, Paul R. LePage of Maine called for action. Seated at a long boardroom table at the Willard Hotel, he erupted in frustration over the state of the 2016 race, saying Mr. Trump’s nomination would deeply wound the Republican Party. Mr. LePage urged the governors to draft an open letter “to the people,” disavowing Mr. Trump and his divisive brand of politics.

The suggestion was not taken up. Since then, Mr. Trump has only gotten stronger, winning two more state contests and collecting the endorsement of Gov. Chris Christie of New Jersey.

In public, there were calls for the party to unite behind a single candidate. In dozens of interviews, elected officials, political strategists and donors described a frantic, last-ditch campaign to block Mr. Trump — and the agonizing reasons that many of them have become convinced it will fail. Behind the scenes, a desperate mission to save the party sputtered and stalled at every turn.