Lets assume a semi-moderate wave hits in 2018 (which I think is more likely than not)... how many California districts do you think will swing to democrats? Given Republican gerrymandering across the country, it is absolutely crucial that Democrats make gains in California.
In a moderate wave, CA-10, CA-25, and CA-49 seem like the most likely to fall, assuming decent recruits. CA-21 would be up there too if Dems got Rudy Salas to run (he even won Kings County this year in his AD race, which Hillary couldn't even pull off). CA-39, CA-45, and CA-48 would all probably require retirements or a 2010-style wave, and their chances of falling I'd say are roughly in that order. D's have a strong bench in CA-39 (Newman and Quirk-Silva), but not really many people of stature in the other two OC districts. Just a few city councilmembers.
PNM got to this first literally while I was writing my post. His is much more detailed, but I largely agree at least with the first 4 seats he mentioned and I don't know enough about the rest. For 10, 25 and 49, I'd honestly be surprised if they
didn't flip, given the ways things are going so far. My reasoning:
If Trump/Republicans continue on this current path/agenda, I personally believe brand damage is inevitable. Incumbent WH parties generally do poorly in midterms even when the incumbent president isn't that unliked, so imagine 2 more years of this behavior/policies from Trump? It might not be negative everywhere, but it's toxic in California.
Part of me believes that Hillary's unpopularity, Trump's absurdity during the campaign and Hillary's efforts to separate him from Republicans helped the GOP brand remain viable for downballot candidates last year, but after 2 years people will almost surely fully associate Trump with the Republican Party, as he is their leader now, regardless of his behavior. Their party now has to take responsibility for his actions and the agenda they are pushing. It's going to alienate people. Even without Trump's incompetent and awful shenanigans, many parts of the GOP agenda were already unpopular with voters.
The reason I pointed that^ out was that additional brand damage will hurt those CA Republicans and help Democrats make up for the turnout drop among minorities/young people.