One of the most influential and wealthiest Republican donors in the U.S. has thrown his support to Florida Senator Marco Rubio. The decision will likely swing millions of contribution dollars behind Rubio at a very critical point in the nominating battle amongst Republicans.

The donor, Paul Singer, a billionaire investor from New York, is a signal victory for Rubio in his battle with Jeb Bush his biggest rival for the affections of major patrons in the GOP and the business wing of the party.

The support comes as a huge blow to Bush, who is seeing a once vigorous campaign imperiled with doubts amongst supporters and whose dominance early in the race was driven by his own financial muscles.

Bush and a number of other candidates, including Chris Christie the Governor of New Jersey had competed for the blessing of Singer.

In a letter by Singer sent to dozens more donors Friday, he described Rubio as the only candidate who is able to navigate the complex primary process and remain in position to defeat Hillary Clinton in the general election.

Singer praised Rubio’s optimism message about the future of the U.S., his work on the Foreign Relations Committee of the Senate and his ability to make a strong case to voters as key reasons for supporting him.

Singer wrote that Rubio was accustomed to thinking of foreign policy in the U.S. as a responsible policy maker.

A spokesperson for Rubio welcomed the Singer endorsement saying our campaign knows there is a great deal to be done before the nomination is won by Marco, but clearly the support by Singer moves everything in the right direction.

Singer gave more money to GOP candidates and causes in 2014 than any single donor.

He is courted by Republicans for both how deep his pockets are to the wide network of conservative givers he influences.

Singer is known for careful and cautious vetting of candidates and even though he is passionately pro-Israel as well as a supporter of same-sex marriage he generally is seen as a donor who does not believe in the litmus tests.

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