The endorsement comes as Crist, once a rising star in the GOP, considers bolting from the party and running as independent in the race. Crist, the state's governor, trails the conservative Rubio by double digits in recent polls. Rep. Kendrick Meek is the likely Democratic nominee.
Polls suggest Crist could win as an independent in a three-way race.
Top Republicans are pressuring Crist to quit rather than run as an independent.