This is the double agent I'm talking about. Why can't democrats do that with Republicans? Why is it always Republicans who run their agents in? They did it with lieberman a few years back too.
Because the barrier for acceptance is pretty low for Democrats right now. The current loudest voices in the Republican party are so far right that even a slightly right leaning centrist could easily gain acceptance. Going the other way, there seems like there's always something new happening where people in the party are essentially being asked to reaffirm just how down for the cause they are. It's hard to be a double agent when you're constantly being forced to reestablish your cover.