Paul Singer’s decision comes as a major blow to Jeb Bush, who is seeing his once vigorous campaign imperiled by doubts among supporters.

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One of the wealthiest and most influential Republican donors in the country is throwing his support to Sen. Marco Rubio of Florida, a decision that could swing millions of dollars behind Rubio at a critical point in the Republican nominating battle.

The decision by the donor, Paul Singer, a billionaire New York investor, is a key victory for Rubio in his battle with Jeb Bush for the affections of major Republican patrons and the party’s business wing.

It comes as a major blow to 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. Bush and several other candidates, including Gov. Chris Christie of New Jersey, had competed fiercely for Singer’s blessing.

In a letter Singer sent dozens of other donors Friday, Singer described Rubio 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 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 policymaker,” Singer wrote. “He is ready to be an informed and assertive decision-maker.”

Singer, who gave more money to Republican candidates and causes last year than any other donor, according to the Center for Responsive Politics, is courted by Republicans for the depth of his 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.

Singer also provides something some other coveted Republican donors do not. Unlike Sheldon Adelson, a fellow Republican billionaire and Israel supporter, Singer is an assiduous and effective “bundler” for candidates: In the 2012 campaign, he raised more than $3 million in the primaries for Mitt Romney, the eventual Republican nominee. Many other donors, particular in the New York financial world, turn to Singer’s political advisers for strategic guidance on their own donations.