Hedge-fund managers made a big bet on Barack Obama and other Democrats in 2008. Now, with the 2012 contest gearing up, some prominent fund managers have turned their backs on the party and are actively supporting Republicans.

Daniel Loeb, founder of Third Point LLC, was one of the biggest Obama fund-raisers in 2008, rounding up $200,000 for him, according to campaign-finance records. In the decade prior, Mr. Loeb and his wife donated $250,000 to Democrats and less than $10,000 to Republicans.

But...