The endorsement is a major boost for the Florida senator's campaign and is a setback for Jeb Bush, whose campaign has far outraised his rivals this year. The news comes days after Bush's much-criticized debate performance and could raise further concerns about his candidacy.
Singer is one of the GOP's leading bundlers, and is able to easily invest millions of dollars into a super PAC. He is likely to make a large contribution to one of the outside groups allied with Rubio's presidential campaign. And it could help Rubio's official campaign -- which only raised $6 million in the most recent fundraising quarter -- raise the cash necessary to challenge Bush.
In a letter to donors obtained by CNN, Singer, who backed Rubio during his Senate campaign in 2010, praised the senator's positions on higher education and welfare reform, his pledge to replace Obamacare and his support for Israel. Singer also called Rubio "the best explainer of conservatism in public life today" and the candidate best positioned to defeat Hillary Clinton next fall.
"In a field full of promise, but also of risk for the party, Senator Rubio is the strongest choice," the letter concluded.
Rubio said he was "grateful" for Singer's support.
"He's someone that's successful at raising money," Rubio told reporters at a campaign event in Orange City, Iowa. "I'm grateful that, you know, when people donate to us they buy into our agenda, and I'm glad that he has and it will help us with resources. But you know, resources alone are not enough. You have to have the right ideas and the right principles and convince people that you're the right person for the job. But we're grateful to have his help, obviously."
Singer's backing was previously reported by The New York Times
and the National Review
A source familiar with calls the Singer team made told CNN they reached out to several major donors Friday afternoon to let them know that Singer would back the Florida senator. The source said the endorsement was a significant blow to Bush's candidacy, because the billionaire has a vast network of people who will give hard dollars to Rubio and lots of money to his super PAC.
Singer, 71, is perhaps the party's most prodigious fundraiser, and his giving has been motivated primarily by two causes: protection of Israel and other Jewish causes, and support for the gay rights movement -- a position that puts him at odds with many other leading players in the Republican Party.
Singer previously raised money for Mitt Romney in 2012 and Rudy Giuliani in 2008.
In 2014, Singer gave $11.5 million to conservative groups, more than any Republican in the country, according to the Center for Responsive Politics.
Singer's endorsement was ridiculed Saturday morning by Donald Trump, who has at times teased Rubio
on the campaign trail and criticized other Republican presidential candidates for being beholden to big-money donors.
"I see Marco Rubio just landed another billionaire to give big money to his Superpac, which are total scams. Marco must address him as "SIR"!" Trump tweeted.