I present to you all a guide to the great state of CALIFORNIA and important House battles here. Currently, 39 of California's 53 representatives are Democrats, and 14 are Republicans.
There are 5 districts Cook rates as Safe Republican: CA-01 in the rural northeast corner, represented by Doug LaMalfa; CA-08 in the mountain/desert area southeast of the Sierras, which is one of 4 districts in the country per Ballotpedia to not have a Democrat running (two Republicans beat all three Democrats running in the jungle primary, but it's safe R anyway); CA-22, centered in the deplorable-heavy Central Valley and represented by Devin Nunes; CA-23, centered in deplorable-heavy Bakersfield, home to both a recent shooting and House Minority Leader Kevin McCarthy; and, lastly, CA-42, centered around Riverside (who lives in Riverside??), represented by Ken Calvert.
With that out of the way, here are your BATTLEGROUND districts.
Lean Democratic
CA-49, saying GOODBYE to Darrell Issa
Clinton 51, Trump 43
A weird district, with a mix of both southern Orange County and northern San Diego County, split in half by Camp Pendleton, a huge Marine Corps base. Darrell Issa was very nearly taken out in 2016, beating challenger Doug Applegate by less than 2,000 votes, and in 2018 he's seen the writing on the wall and announced his retirement long ago.
Applegate ran again, but the Democrat to emerge from a widely contested primary was Mike Levin, an attorney.
Toss Up
CA-10, currently represented by Jeff Denham
Clinton 49, Trump 46
A widely rural Central Valley district centered on Modesto, this place must be less deplorable than the Fresno area because Josh Harder, a venture capitalist, appears to actually have a chance to win it. His platform is all over the map, he says he wants to work across the aisle but is also pro-M4A.
Beat: apparently CPHoya and I share our alma mater with deplorable Denham.
CA-25, currently represented by Steve Knight
Clinton 50, Trump 44
This district north of LA winds across a few disparate areas - Simi Valley, the Santa Clarita Valley, and the Antelope Valley cities of Lancaster and Palmdale which tbh feel like an entire world away (unlike the other areas of this district) from LA if you've ever driven through them. Knight's opponent, Katie Hill, is an executive director of a homelessness nonprofit and has raised over $2 million. And, come on, this seems like a pretty easy choice:
CA-39, saying GOODBYE to Ed Royce
Clinton 52, Trump 43
This district straddles northern Orange County and southeastern LA County, and like Darrell Issa, Ed Royce peaced the **** out when he figured out what was coming this year.
Gil Cisneros, a former Republican and veteran turned lottery winner, somehow emerged from a crowded field with better candidates to get the D nomination. His opponent is Young Kim, a former Royce staffer who seems like a weird choice to continue his legacy in a year where he retired expecting to get slammed. Cisneros has raised $5m but notably does not appear on the coveted Obama list of endorsements for tight CA races.
CA-45, currently represented by Mimi Walters
Clinton 50, Trump 44
This is a rapidly changing district, as Santa Ana becomes drastically more Hispanic in contrast with other traditionally conservative and wealthy areas of Orange County. Walters is being taken on by Katie Porter, a law professor at UC Irvine. They've both raised $2-3m and polls show a tight race.
CA-48, currently represented by Dana "paid by Putin" Rohrabacher
Clinton 48, Trump 46
Some similar demographic trends to CA-45 above, but also with the more famous and hyper-wealthy areas of Orange County in this district like Laguna Beach. His opponent is businessman Harley Rouda, who's raised over $3m.
Lean Republican
CA-50, currently represented by Cool Guy Vaping Congressman Duncan Hunter
Trump 55, Clinton 40
This one wasn't really on the map, having been represented by the Hunter dynasty (both the current Duncan Hunter and his father, also Duncan Hunter) for decades, until indictments came down showing that Hunter treats his campaign donations like a piggy bank for family trips to Italy. As mentioned in the OP, his handsome progressive opponent is Ammar Campa-Najjar, who has his work cut out for him in a district that borders Issa's to the west, but does not border the ocean and thus is that much more deplorable.
Likely Republican
CA-04, currently represented by Tom McClintock
Trump 54, Clinton 39
A huge district geographically, covering Lake Tahoe and the Sierras all the way down through Yosemite and Kings Canyon, whose population is mostly centered on Sacramento suburbs. His opponent is Jessica Morse, who describes herself as a 5th-generation Northern Californian who worked as a national security strategist. Trying to find more about her led me to her campaign website which has
some one of the most bland, nondescript issues pages I've ever seen, but maybe being perfectly milquetoast is just what this district needs? idk, probably not. Morse is the other candidate in these 9 contested seats to
not get an Obama endorsement.
CA-21, currently represented by David Valadao
Clinton 55, Trump 40
A
very rural Central Valley district, whose largest city is Hanford (population 55k). The district has a cook PVI of D+5, I guess owing to it being 71% Hispanic, but Valadao's opponent is a businessman who doesn't live in the district (he elected to run in CA-21 instead of against Josh Harder in CA-10, which was one of the "oh **** are Dems gonna **** themselves in the jungle primary" districts) and is not regarded as a strong candidate to beat Valadao. We'll have to see if the large anti-Trump current this district demonstrated in 2016 is enough to oust Valadao on its own.