As I see it, he has two options. First, he can bow out of the primary, campaign energetically for Rubio in November, and hope the party moves to the center far enough for him to run again at some future date. Second, he can make the case that the party has gotten too extreme for him — a legitimate case, as Crist is genuinely moderate on most of the key issues — and run for Senate as an independent or as a Democrat. Neither option is particularly easy, though the second seems easier than the first.
And now it looks like Crist just might be thinking along the same lines. Apparently he plans to join President Obama for his political appearance this week in Tampa.