Yeah, but what will they do? Someone like Booker will probably want to attack Trump from a "he's a crazy racist, I mean how could you support him?!" standpoint, and we saw how that worked for Hillary. Someone like Bernie would want to attack him from a "this guy says Populist Thing X and Populist Thing Y, but he's governed like just another Republican giving tax breaks to billionaires and corporations, and he doesn't care about people like you." I am inclined to believe - specifically with respect to downballot races - the second strategy is much, much more effective.
Obviously the climates were different, but in 2004 Democrats more or less ran on the idea that Bush was a semi-retarded frat boy in the White House, and we needed to elect someone with a functioning IQ like Kerry ... in 2006, they hammered Bush as an imperialist Wall Street crony who sent hardworking Americans' kids to go die for oil ... one worked a hell of a lot better than the other.
Nobody liked Hillary though. Had Hillary been elected, the GOP would of won a super majority in 2018. Hillary was totally loathed by the Bernie wing of the party even more than the Republicans hated her.
I think this will end in one of 2 ways:
1) Trump is where Carter was in 1976. Disliked by the opposing party yet not trusted or liked by his own party; on the cusp of an emerging realignment that hasn't quite been figured out yet. Trump may fail miserably at things totally our of his control like Carter did or just flat out fail to lead.
-or-
2) Trump is where Reagan was in the 80s. Meaning that he will do a decent job as president but the opposing party will make tons of gains and keep him in check while they figure out who/what their party is about.
Both parties are fractured and your seeing splits emerge even right now. Alot of the neocon faction of the GOP endorsed Hillary. If Trump pals up with Russia, your going to see that part split and possibly join with the Democratic coalition. Trump has also turned off a decent amount of republicans from the idea that the free market can solve every problem. They now dislike free trade and are warming up the idea that health care is a right. If Trump fails to deliver on these things then that's a wing of the GOP that can be split and taken by the Democrats in 2020, much like the GOP took blue collar Democrats in 1980.
The thing is, Democratic politicians and Democratic voters (outside of teens on Atlas and self-absorbed "pundits" on CNN) don't want to alter their policies in a way that would appeal to those voters. Moderate/business/affluent/neocon/whatever Republicans might not like Trump, but their alternative is worse, and they voted accordingly in 2016. I highly doubt the Democrats will move in a direction that appeals to them, and so far they've done the opposite.
And Rocky Republicans of the Northeast didn't want to change things much to appeal to Dixiecrats either. They even put up John Anderson to try and stop it. But the "failures" of Carter compelled 'em.
Look what happened.
And as for '04, Bush was also on trial for "Mission Accomplished"...would've worked if there wasn't the Swiftboat and Kerry's "I was for it before I was against it" line. It wasn't as simple as "Bush is an idiot".
