If my Florida friends don't already have their Marco Rubio yard signs, I hope you'll order 'em now. I don't always hate it when a politician changes parties (I've written more than one email to my congressman, Bobby Bright, urging him to do that very thing). I think, however, that it is an unforgivable sin in two instances.
First, if it changes the balance of power in the House or Senate. Supporters should be respected enough that their votes should never be used to give the other party control in this fashion. It's one thing to lose a player to free agency and another to lose the whole season. Arlen Specter performed a subset of this by switching parties and giving the Dems a filibuster-proof majority in the Senate (after Al Franken was sworn in and until Scott Brown was elected).
Second, when it's clear that a politician is going to lose a primary. It smacks of sore loserness and demonstrates an "I'm going to take my marbles and go home" sort of attitude that should create a flashing neon warning sign for voters. Politics ain't beanbag. Sometimes you win, sometimes you lose. You might scream into your pillow at night but during the day you put on a happy face, support the winner, and play good party soldier.
Obviously, the first situation is worse than the second. In the first, voters cast ballots for a white dog and end up getting a black dog for their troubles. In the second, voters at least know what they're doing. But I still don't have to like it.
The pundits seem confident that Crist will lose; I know how funky Florida can be so I'm a little less sure. He might siphon enough moderate votes away from Rubio that Florida could end up sending an "independent" Crist to the Senate (or worse, a Democrat). Would he caucus with the Republicans, as Joe Lieberman does with the Dems? Maybe--but he's coming to the party with "RINO" stamped all over him (and he wouldn't even have the "NO" part of that label).
So beware, Florida voters. You've got a self-serving politician out for his own self interests. Yeah, I know. Kind of describes most of 'em...