@evan (I selected "Mix")
"By party" answers really frustrate me, because:
Where I'm from (Indiana):
1) With the way the state D party runs, it's all but impossible to primary against an incumbent D. So you if the D is running for reelection, but you think they're doing a shit job, the most realistic thing to do is run as an R. (I imagine the converse is true too, but am not familiar with how the state R party is run.)