“A Cruz-Kasich ticket is the best way we can stop Trump,” he said. “It’s time to think big here.”

If they will not run together, said Mr. Graham, they at least need to form “an alliance” in which they divvy up states to not drain votes from each other — something neither seems willing to do. “The main thing is we’ve got to get into the convention with Trump under 1,237 delegates.”

Yet other top Republican officials are not nearly as dedicated to plotting this or any strategy to deny Mr. Trump a majority before the party gathers in July in Cleveland.

Even those who have signaled they are likely to support Mr. Cruz are dragging their feet. Gov. Scott Walker of Wisconsin, who has spoken favorably of Mr. Cruz, said he would not announce his choice until closer to his state’s April 5 primary — for “maximum impact.”

Mr. Rubio, who praised Mr. Cruz as “the only conservative left in the race” after his exit, has been perhaps the most conspicuous holdout. He is one of the few Republican senators who gets along well with Mr. Cruz, and the two have been in touch since Mr. Rubio withdrew from the race.