Indispensable reporting from Burns, Haberman, and Martin:

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.”

“We want voters to imagine Donald Trump in the Big Chair in the Oval Office, with responsibilities for worldwide confrontation at his fingertips,” they wrote in the previously unreported memo. Mr. Castellanos even produced ads portraying Mr. Trump as unfit for the Oval Office, according to people who saw them and who, along with many of those interviewed, insisted on anonymity to discuss private conversations.

The two strategists, who declined to comment, proposed to attack Mr. Trump in New Hampshire over his business failures and past liberal positions, and emphasized the extreme urgency of their project. A Trump nomination would not only cause Republicans to lose the presidency, they wrote, “but we also lose the Senate, competitive gubernatorial elections and moderate House Republicans.”

No major donors committed to the project, and it was abandoned. No other sustained Stop Trump effort sprang up in its place.

Resistance to Mr. Trump still runs deep. The party’s biggest benefactors remain totally opposed to him. At a recent presentation hosted by the billionaires Charles G. and David H. Koch, the country’s most prolific conservative donors, their political advisers characterized Mr. Trump’s record as utterly unacceptable, and highlighted his support for government-funded business subsidies and government-backed health care, according to people who attended.

But the Kochs, like Mr. Adelson, have shown no appetite to intervene directly in the primary with decisive force.