Add to the above, that already democrats have more than twice the number of incumbents who've chosen not to run again than Republicans who are opting out in the upcoming mid-terms. Dems know that there's a much greater chance of keeping a seat if someone is running for re-election.
Things aren't looking good for democrats at this point. Situations can always change, but I just don't see them getting out of this one. They went sprinting to the left and went over a cliff, so I don't see them climbing back anytime soon.