Asked why he opted for Rubio instead of Bush, Griffin told CNBC, "I think that Marco Rubio has the vision of where America needs to go."
In an e-mailed statement, Alex Conant, a spokesman for the Rubio campaign, said: "We greatly appreciate his support as we enter a critical phase of the campaign."
Griffin cited Rubio's relatively hawkish stance on foreign policy as well as his smaller-government approach to domestic policy as two of the candidate's attractions. Rubio "will certainly strengthen our military and work to ensure the safety of our people," said the Citadel chief, adding that "at home, he will lift the weight of the government from our shoulders, in everything from free enterprise to choice in health care." Among Griffin's list of specific concerns, he said, was the fact that major insurers in his home state of Illinois were declining to participate with Obamacare.
As part of his candidacy, Rubio has argued for a simplified tax code with lower rates that includes higher tax credits for children under 16, a measure that has been controversial among conservatives. He has also been open about his own past financial struggles, which included needing to liquidate a $68,000 retirement account at one point to purchase a new refrigerator, among other things.
Griffin, an entrepreneur who founded Citadel from a college dormitory in 1990, said he was unfazed by Rubio's financial history. "It's hard to find somebody in the U.S. who hasn't had in their lifetime at one point or another financial challenges," Griffin said. "If these are the issues we're talking about as points of concern, I've got the right guy."
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He said he had shared his plans with Rubio in a phone call Wednesday morning, saying that "I'm supporting you not just because you can win, but because I believe in the values that you represent."
His decision falls a month and a half after another influential hedge-fund manager and Republican donor, Paul Singer, embraced the Florida Senator.
Griffin told CNBC on Thursday that while he admired Singer's track record as a money manager, he had decided to back Rubio "independently."
With a net worth estimated by Forbes to be $7 billion, Griffin is thought to be the richest person in Illinois, so depending on the level of financial support he provides, he could be crucial to a Rubio candidacy. In 2014, for instance, Griffin helped secure a gubernatorial victory for private-equity executive Bruce Rauner in Illinois by contributing $5.5 million and reportedly offering the use of his private plane.
In the telephone interview Thursday, Griffin said he would play an active role in raising money for Rubio from his own network of associates. He also said he would contribute "several million dollars" to Rubio's PAC starting "imminently."