Can't they do that now? If I think my chosen primary guy is winning in a landslide I could just register for another party I don't like and vote for someone who I think is easier to beat.
You would still forfeit the ability to vote in your primary though. I do think there are people that do this, but most people want to vote in their primary regardless of whether it's a landslide.
I envision that it does not matter, because this is a tactic that would 1) be available to all, and 2) it gives up your vote for someone of your own party, thereby weakening your own position. It's self regulating.
