A lot of Democrats just before the election in 2006 were suggesting that if they couldn't even win the House, there would simply be more very liberal Congresscritters going the way of Bernie and becoming Independent or Green.
I mean, that would be silly. Why abandon the Democratic Party for a party that is even less successful
(by far)? Lots of people have these dreams and fantasies of making other parties relevant, but besides an election or two at the state/local level, they hardly ever make a difference. Our system is built for 2 parties, and the Democratic Party is simply too well known and too ubiquitous to be supplanted. It
could happen, but it would have to be some cinema-esque situation that just comes together perfectly "against all odds," as the trope goes.
I even heard Charlie Cook say something to the effect that if they didn't win the Senate, they were going to fire a bunch of people and if they didn't win the House, they would "go the way of the whigs".
Pundits and professional prognosticators love saying that stuff. I still stand by my point that both parties have been in worse situations and still toiled their way through hard times. Look at the post-Civil War era - the Democratic Party was a minority party at the federal level for almost 72 years, and while they had some success in the House and a few odd wins here and there in terms of Senate majorities / White House, it was mostly bleak. Republicans got hollowed out during the New Deal and they still bounced back. So yeah, comparatively, the Democratic Party isn't in that bad of a position.
I have a good feeling that by 2019, people will be singing a different tune. Republicans had an often-unpopular Democratic president they could milk for midterm waves, and now the shoe is on the other foot.