WASHINGTON—Florida Gov. Charlie Crist, determining he cannot win the Republican primary for U.S. Senate, has decided to exit the race and run instead in the general election without a party affiliation, according to two advisers.

The governor has scheduled a formal announcement for 5 p.m. Thursday at a waterfront park in his hometown of St. Petersburg, Fla.

A campaign official declined Wednesday afternoon to discuss Mr. Crist’s plans.

The advisers, both of whom said they spoke with Mr. Crist Tuesday night, said the governor was also planning a major fund-raiser for Sunday in Miami to try to keep most of his big donors from fleeing the campaign.