Mitt Romney’s exploration of a third presidential campaign ended Friday after three tumultuous weeks of deliberations that led him to conclude that, while he might emerge with the Republican nomination again in 2016, he might be so badly wounded in the process that he would have trouble defeating Hillary Rodham Clinton in a general election.

Romney’s sudden decision to declare his interest had been prompted by his concerns over rival Jeb Bush’s aggressive moves to poach from his 2012 coalition, according to intimates. It was fueled further by a mountain of polling data commissioned earlier for one of his donors — suggesting Romney was in the strongest position of any Republican.

But by the end of last weekend, Romney had made the call, according to those familiar with the deliberations. He sat on it for a few days to assure himself that not running was the right thing to do for himself and his party before making his announcement Friday. He bowed out in a call to supporters still convinced he was as well-qualified as anyone in the party to be president.

His decision, he said, was aimed at giving other, lesser-known candidates the opportunity to emerge and eventually prosper. “After putting considerable thought into making another run for president, I’ve decided it is best to give other leaders in the party the opportunity to become our next nominee,” Romney said.

Romney’s decision spares the Republican Party a potentially bruising battle between its past nominee and its rising stars. It poses an opportunity not only for former Florida governor Bush, but also for New Jersey Gov. Chris Christie, Wisconsin Gov. Scott Walker and other GOP presidential aspirants.

Mitt Romney tells supporters in a conference call that he will not run for president in 2016. (The Washington Post)

The past three weeks reminded Romney of just how grueling and potentially damaging another nomination contest could be, confidants said. From the time he announced that he was seriously considering another campaign, he became the target of criticism from inside the GOP, from Democrats and from others.

“It’s been all trained on him, and I think that would continue to be the case for the next year,” said Tagg Romney, the former governor’s eldest son, in an interview. “His hope is that someone else can come out and catch fire.”

Another Mitt Romney adviser said, “He went into it intellectually knowing he would have to earn it again — not that he’d forgotten it. But memory had blurred some of those sharp edges and it came into sharp relief.”

In announcing his decision, Romney told supporters, “I am convinced that we could win the nomination, but fully realize it would have been difficult test and a hard fight.”

In many ways, Romney’s January roller coaster of conference calls, political outreach and data analysis was as much about Bush as it was about anything else. Bush’s early moves forced Romney’s hand.

Romney was warned this month that, unless he acted to show interest in another campaign, there could be little left of the financial and political network that carried him to the nomination in 2012.

But Romney associates also say that, had Bush sought Romney’s advice and support early on, Romney might never have been stirred to action. Instead, Bush’s aggressive efforts to recruit donors and former staffers from Romney spurred on the former Massachusetts governor.

“It was like poking a bear,” one Romney associate said.

Another Romney adviser said the 2012 nominee was less annoyed by Bush than simply worried that his move might preclude even consideration of another campaign. “Mitt realized that if he was going to consider this, given the acceleration, he needed to move fast,” said the adviser, who along with others declined to be identified in order to speak candidly about internal discussions.

During the deliberations, Romney and Bush met once at Romney’s Utah home on Jan. 22. Romney arrived armed with a mountain of polling data that his team had collected, which seemed to shape his view of the race. Romney shared the information as a way of explaining his motivation to think about running again.

A former Romney campaign donor had commissioned an extraordinarily robust portfolio of private polling data culled from interviews this month with thousands of voters in some 20 states. “The level of support was broad and deep,” a Romney associate said.

According to these accounts, Romney and Bush talked about the race and about some of the relevant issues over lunch. Both camps said it was cordial and mostly social. How it may have shaped Romney’s thinking is unclear.

Tensions had been running high between the Romney and Bush camps for weeks, and in his Friday remarks, Romney offered an assessment of the 2016 race that some saw as an indirect swipe at Bush, who is well-known nationally as the son and brother of former presidents.

“I believe that one of our next generation of Republican leaders — one who may not be as well-known as I am today, one who has not yet taken their message across the country, one who is just getting started — may well emerge as being better able to defeat the Democrat nominee,” Romney said. “In fact, I expect and hope that to be the case.”

Bush wrote in a Facebook post on Friday that Romney “has been a leader in our party for many years” and that “there are few people who have worked harder to elect Republicans across the country than he has.”

The Romney camp suffered a major defection this week with the news that David Kochel, Romney’s point person in Iowa, would be joining Bush, likely as national campaign manager.

The Kochel decision shook some in the Romney campaign — and stung as well. According to Romney sources, he was on a number of strategy conference calls with other Romney senior advisers and had encouraged Romney to run again. One Romney associate said the team sees Kochel’s defection as a Benedict Arnold moment.

Kochel, in a telephone interview Friday, said, “It’s incorrect to say I urged him to run and I didn’t say I would not go to work for Gov. Bush. . . . I have tremendous respect and affection for Gov. Romney and his family.” Once he received a direct offer from Bush, Kochel said, he stopped participating in Romney deliberations and personally told Romney of his decision.

Romney openly had been weighing a 2016 run since telling a group of former campaign donors in New York on Jan. 9 that he still wanted to be president.

One issue that seemed to weigh on Romney was the Jan. 7 terrorist attack in Paris on the Charlie Hebdo publication. Romney talked about the issue with close advisers the night before he declared he would seriously consider running.

“Paris was the biggest of all the factors,” the Romney associate said. “It was a tipping point for him about how dangerous the world had become.”

Romney’s exploration invited a barrage of critical reaction from many Republican leaders, conservative commentators and major donors.

Romney’s advisers had discounted the impact of such criticism, saying Romney was gathering data, speaking to as many people as possible and then weighing the evidence.

The personal toll another race would take on his wife, five sons and daughters-in-law and 23 grandchildren factored into his decision, advisers said.

“As much as you might want to be the candidate, you sometimes realize you can be more effective at helping fulfill a different role,” said longtime Romney adviser Ron Kaufman. “He’s an amazing person and he doesn’t need to have the captain’s seat.”

Another Romney confidant, former Utah governor Michael Leavitt, said Romney wanted to run but concluded it was better for his party and for the country to allow a younger generation of Republicans emerge as leaders.

“In the cynical world of politics, it is unlikely that anyone will understand the sacrifice that this represents for a man who clearly believes he could run and win and contribute,” Leavitt said.

Romney’s decision to forgo a third run came after a lengthy meeting of Romney’s inner circle in Boston last Friday, during which they evaluated feedback from former campaign donors and activists in key early voting states.

The assessment was realistic — “we were not Pollyannaish,” one adviser said — and included reports from Iowa, New Hampshire and South Carolina, where Romney would have lost some key precinct leaders but still had considerable support.

Many participants left that session convinced that it was all but certain he would run again.

“It’s a very personal decision,” said a senior adviser, who like others interviewed requested anonymity to speak candidly. “All the political metrics were positive. Ultimately, running for president, you just have to feel right about it in your heart. They just didn’t feel it was right. He’s a happy person. He’s not a needy, desperate guy.”

Another adviser said Romney had some regrets about not running again but was certain he made the right decision, adding, “If he hadn’t gone through this difficult three weeks, his regret would have been greater.”

Karen Tumulty and Matea Gold contributed to this report.