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“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. …

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.

Efforts to unite warring candidates behind one failed spectacularly: An overture from Senator Marco Rubio to Mr. Christie angered and insulted the governor. An unsubtle appeal from Mitt Romney to John Kasich, about the party’s need to consolidate behind one rival to Mr. Trump, fell on deaf ears.

At least two campaigns have drafted plans to overtake Mr. Trump in a brokered convention, and the Senate majority leader, Mitch McConnell of Kentucky, has laid out a plan that would have lawmakers break with Mr. Trump explicitly in a general election.” …

Late last fall, the strategists Alex Castellanos and Gail Gitcho, both presidential campaign veterans, reached out to dozens of the party’s leading donors, including the casino magnate Sheldon Adelson and the hedge-fund manager Paul Singer, with a plan to create a “super PAC” that would take down Mr. Trump. In a confidential memo, the strategists laid out the mission of a group they called “ProtectUS.” …

While still hopeful that Mr. Rubio might prevail, Mr. McConnell has begun preparing senators for the prospect of a Trump nomination, assuring them that, if it threatened to harm them in the general election, they could run negative ads about Mr. Trump to create space between him and Republican senators seeking re-election. Mr. McConnell has raised the possibility of treating Mr. Trump’s loss as a given and describing a Republican Senate to voters as a necessary check on a President Hillary Clinton, according to senators at the lunches.

He has reminded colleagues of his own 1996 re-election campaign, when he won comfortably amid President Bill Clinton’s easy re-election. Of Mr. Trump, Mr. McConnell has said, “We’ll drop him like a hot rock,” according to his colleagues.”