One of the wealthiest and most influential Republican donors in the country is throwing his support to Senator Marco Rubio of Florida, a decision that could swing millions of dollars in contributions behind Mr. Rubio at a critical point in the Republican nominating battle.
The decision by the donor, Paul Singer, a billionaire New York investor, is a signal victory for Mr. Rubio in his battle with his rival Jeb Bush for the affections of major Republican patrons and the party’s business wing.
It comes as a major blow to Mr. Bush, who is seeing his once vigorous campaign imperiled by doubts among supporters, and whose early dominance of the race was driven by his financial muscle. Mr. Bush and several other candidates, including Gov. Chris Christie of New Jersey, had competed fiercely for Mr. Singer’s blessing.
In a letter that Mr. Singer sent to dozens of other donors on Friday, which was obtained by The New York Times, Mr. Singer described Mr. Rubio — who was elected to the Senate in the Tea Party wave but has been embraced by the party’s Washington elite — as the only candidate who can “navigate this complex primary process, and still be in a position to defeat” Hillary Rodham Clinton in a general election.
He praised Mr. Rubio’s message of optimism about America’s future, his work on the Senate Foreign Relations Committee and his ability to make a persuasive case to voters as key reasons to support him.
“He is accustomed to thinking about American foreign policy as a responsible policy maker,” Mr. Singer wrote. “He is ready to be an informed and assertive decision-maker.”
Alex Conant, a spokesman for Mr. Rubio, welcomed the endorsement, adding, “We know we have a lot of work to do before Marco wins the nomination, but clearly this moves us in the right direction.”
Mr. Singer, who gave more money to Republican candidates and causes last year than any donor in the country, according to the Center for Responsive Politics, is courted by Republicans both for the depth of his own pockets and for his wide network of other conservative givers. He is known for his caution and careful vetting of candidates and, while passionately pro-Israel and a supporter of same-sex marriage, he is generally viewed as a donor who does not believe in litmus tests.
In recent years, he has frequently deployed his network to cultivate up-and-coming Republicans who he believes can help expand the party’s demographic appeal. Among them are Representative Elise Stefanik of New York, the youngest woman ever elected to Congress, and Mr. Rubio himself, whom Mr. Singer backed early in his 2010 race when many of Mr. Singer’s peers sided with Charlie Crist, then Florida’s Republican governor — a moment that was seen as a turning point in the race.
The battle for Mr. Singer’s support — which included months of behind-the-scenes lobbying by aides and appearances by candidates over the last year at dinners and breakfasts convened by Mr. Singer — underscores the growing clout of big donors in presidential elections, particularly this year, when “super PACs,” and the wealthy donors who finance them, have moved to the center of the race.
But Mr. Singer provides something that some other coveted Republican donors do not. Unlike Sheldon Adelson, a fellow Republican billionaire and Israel supporter, Mr. Singer is an assiduous and effective “bundler” for candidates: In the 2012 campaign, he raised more than $3 million to try to help elect Mitt Romney, the eventual Republican nominee. Many other donors, particular in the New York financial world, turn to Mr. Singer’s political advisers for strategic guidance on their own donations.
And Mr. Rubio, who struggled to raise campaign cash over the summer and has relied heavily on outside groups to pay for advertisements promoting him, needs their help.
Both Mr. Rubio and Mr. Bush eagerly sought Mr. Singer’s backing, as did Mr. Christie, and all three have ties to the wealthy hedge fund manager.
Mr. Rubio has aggressively embraced the cause of wealthy pro-Israel donors like Mr. Adelson, whom the senator is said to call frequently, and Mr. Singer, who both serve on the board of the Republican Jewish Coalition, an umbrella group for Republican Jewish donors and officials. Mr. Bush has been less attentive, in the view of some of these donors: Last spring, he refused to freeze out his longtime family friend James A. Baker III, the former secretary of state, after Mr. Baker spoke at the conference of a liberal Jewish group.
The lobbying of Mr. Singer intensified in recent weeks as Mr. Bush’s debate stumbles and declining poll numbers drove many donors to consider Mr. Rubio anew. Last week, Mr. Bush’s campaign manager, Danny Diaz, and senior adviser, Sally Bradshaw, flew to New York to make personal appeals on Mr. Bush’s behalf, in the hopes of heading off an endorsement of Mr. Rubio, according to two people close to the former governor’s campaign.
But Mr. Singer had been leaning toward Mr. Rubio, and there was no single moment that convinced him, these people said. It was time to make his support known. Mr. Singer, according to people familiar with his thinking leading up to the endorsement, takes his time weighing an endorsement in presidential races, after making an early commitment to Rudolph W. Giuliani in the 2008 race and seeing his candidate falter.
In his letter to his donor network, Mr. Singer described Mr. Rubio as “the best explainer of conservatism in public life today, and one of the best communicators the modern Republican Party has seen. Marco Rubio can appeal to both the head and the heart.”
Of the roughly 1,200 people who raised money or hosted fund-raising events for Mr. Romney in 2012, according to a New York Times analysis, about two-thirds had yet to give a donation to any of the Republican candidates through the end of September, the most recent disclosures available from the Federal Election Commission.
Mr. Bush and Mr. Rubio have competed fiercely in the money race, targeting many of the same donors and bundlers, especially in Florida, where Mr. Rubio serves as senator and Mr. Bush was governor. Some Bush donors said privately that they originally joined Mr. Bush’s operation out of respect for and loyalty to the former Florida governor, despite a belief that Mr. Rubio might have been the party’s better bet. Now they are expressing concern about Mr. Bush’s attacks on Mr. Rubio, saying Mr. Bush’s direct swipe during the debate at Mr. Rubio’s character and credibility was dangerous for both men.
Mr. Bush’s stilted debate performances have set off a new round of jockeying as Mr. Rubio’s supporters seek to lure some Bush backers to their camp. Several people involved in Mr. Rubio’s fund-raising said they had been fielding calls from Bush donors since Wednesday’s debate, suggesting they were rethinking their decision.
“I don’t know if you’ll get a tsunami of people immediately, because these are good people, and they are loyal,” said Jonathan Burkan, a New York financial executive who is supporting Mr. Rubio. “But you’ll get some people.”
Among Mr. Bush’s supporters after the debate, the tone was often one of despair, but it was rarely projected in public comments. Several of them said they took heart that Mr. Bush had acknowledged that he needed to be a more artful political performer in the weeks ahead.
In a telephone interview, the donor and former ambassador under Presidents George H.W. Bush and George W. Bush, Mel Sembler, said that the super PAC supporting Mr. Bush, Right to Rise, is holding strong with Florida supporters.
But will people outside Florida decamp from Mr. Bush’s side? “I hope not,” he replied.
An earlier version of this article incorrectly stated when Paul Singer raised more than $3 million to try to help elect Mitt Romney. The money was raised during both the primary and general election in 2012, not just the primary.