Mr. Bush’s strategy, mainly behind the scenes, was smooth and effective, as he reached out to potential supporters, mixing wonky ideas with personal charm. In a December telephone call with Joe Craft and Kelly Knight, the husband and wife who are coveted contributors in Kentucky, Mr. Bush offered his views on business and education and seemed to have all the time in the world for the couple, who raised millions for Mr. Romney’s 2012 candidacy. And, in a follow-up email, he flattered the pair, who are also the largest financial boosters of the University of Kentucky basketball program, with his careful attention to their passion.

“Saw the Wildcats did pretty well on Saturday,” Mr. Bush wrote, after Kentucky thrashed UCLA by nearly 40 points. “Congrats.”

The former Florida governor’s supporters were not as gentle as they made the case to Mr. Romney’s former backers. They argued that, because of his awkward persona and business background, Mr. Romney could not win a general election, had no rationale for a third campaign and would not be as strong as Mr. Bush in such battleground states as Florida and Colorado.

On Jan. 23, about two dozen of his advisers gathered in Boston for a cleareyed assessment of Mr. Romney’s challenges. “It was, ‘Let’s cut the cheerleading rah-rah and talk about what it is that you know and what it is that you’re hearing,’ ” said someone with knowledge of the meeting, speaking anonymously to talk about a private discussion. “Could this thing get going again?”

Mr. Zwick said he felt confident that Mr. Romney could raise the $50 million to $100 million necessary for a primary, but the softening support among contributors emerged as a concern. Even if he prevailed in a primary, he would be battered and bruised by the general election. And they puzzled over how to appease donors, still frustrated from the stumbling 2012 campaign, who were demanding a new message and a shake-up of advisers.

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.

But by then it was clear the political world was already moving on. In a particularly stinging blow, David Kochel, who oversaw Mr. Romney’s Iowa caucus campaigns in 2008 and 2012, had signed on to manage Mr. Bush’s campaign. That added to the bitter feeling of Mr. Romney’s inner circle toward the Bush operation.