So here's a thought:
The swing from Obama's victory over McCain in Massachusetts in 2008, to Scott Brown's victory over Martha Coakley in the 2010 special Senate election (to fill the vacancy left by the death of Edward Kennedy) was 30.57 percentage points.
A similar swing in California to fill the vacancy left by the resignation of Kamala Harris to assume the vice-presidency would see the Republican candidate (if one makes it to the general in California's top-two jungle primary system) win by around 1.4 percentage points.
You're making the mistaking of thinking that the voting demographics in California follow those of Maine and Mass.
Ironbnite-they don't.
I agree. They don't. (Though California elected a Republican as Governor as late as 2006.)
I'm not saying it's likely. I'm saying that it has happened, and it's not a seat the Democratic Party should take for granted.
EDIT: For a more recent example, the swing in Alabama from 2016 for Trump to 2017 for Jones would still be enough (by about 0.2 percentage points). Of course, Roy Moore was his own special kind of despicable, but there's another example.
EDIT #2:
https://www.cbc.ca/news/world/republicans-impeachment-two-1.5872561One reason more Republicans in the House didn't vote to
convictimpeach (EDIT #3: Thanks for the catch, ironbite) Trump? They were afraid for their physical safety.