“Of everybody I contacted, I only heard from one person who thought Mitt should give it another shot,” said Mr. Oberndorf. In the weeks after he expressed renewed interest in running, Mr. Romney contacted some of his most loyal supporters. But often, he found Mr. Bush had gotten there already.

The former Massachusetts governor called Eric Tanenblatt, an Atlanta lobbyist, who reminded him they had talked the previous month and that Mr. Tanenblatt was supporting Mr. Bush.

It was a recurring theme: Mr. Romney’s supporters had taken him at his word when he had said repeatedly after 2012 that he would not run again…

On Sunday, Mr. Zwick flew to Utah and delivered the message to Mr. Romney: Yes, the team believed Mr. Romney had a path to the nomination, but he faced far more challenges than in 2012, and needed to make up his mind this week. By that point, Mr. Romney and his wife Ann had already decided against another bid, but wanted to “sit with it for a few days to make sure.” By Thursday night, Mr. Romney was ready, phoning his closest friends, family and advisers to tell them what he would make public the following morning: He would not run.