(NEWSER) – An ominous sign for the Jeb Bush campaign: Paul Singer, one of the Republican Party's key donors, has decided to support Marco Rubio instead.

Singer—who was the GOP's biggest donor last year and raised $3 million for Mitt Romney in 2012—praised Rubio in a message to dozens of fellow donors, the New York Times reports. "He is accustomed to thinking about American foreign policy as a responsible policy maker," the hedge fund manager wrote. "He is ready to be an informed and assertive decision-maker."

Rubio is the "best explainer of conservatism in public life today" and can "appeal to both the head and the heart," wrote Singer, describing the senator as the only candidate who can "navigate this complex primary process" and go on to defeat Hillary Clinton.

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