Donna B (I think) mentioned this in a December comment section, and I have pondered it and adopted it. What conservatives call a RINO (and presumably, what progressives call a DINO) may be more a reference to a willingness to compromise than to actual policy positions. Romney's position on immigration, Rubio's position on corporate welfare, Sanders's position on mortgage regulations, are less important to their supporters than the sense that he will/won't cave. That very much ties in to an entire sense of tribal defense, or attack on the correct enemies, than on getting it right.
This changes everything, slightly.
I have wondered if time-travel with perfect knowledge could create a senator who could be elected indefinitely; because after the first two election failures with perfect predictions of what would be most important going forward, the third election would succeed and the fortress would be impregnable. I have decided this would not be so*, not merely from cynicism of the crowd wanting bread and circuses, but because the role of tribal representative is valuable not only for decorative, but for cultural purposes. People live their tribal lives now. They want to protect their posterity and have some sense of what will do that, but they are not wired to respond to much beyond My Tribe, Now.
Update: Donna says it was T99.
*Because John Sununu basically did that and then lost to a social studies teacher who votes the Party Line reliably.