Gavin Newsom (red) vs Antonio Villaraigosa (yellow)
Villaraigosa most competitive in Inland California & among anti-Newsom Republicans (Those that choose not to sit out), but he gets crushed in Northern California & coastal regions. Newsom wins LA County 55-45 and under-performs Harris' Senate win by 1-2%
A D vs. D race can be unpredictable. Who knows what GOP turnout would be like. Would Republican turnout be depressed, or would Republicans have normal turnout and reluctantly vote?
Even if it's D v D for the gubernatorial and senatorial races, as long as there's some red meat in the ballot measures, conservatives will still show up (which is good for some of the more vulnerable House members). However, just because they show up and vote doesn't mean that they won't just leave a field blank. Large numbers of conservative voters did just that in the 2016 senate race, which was D v D.
Numbers from Atlas:
Modoc President 3,843
Modoc Senate 2,575
Lassen President 10,699
Lassen Senate 7,377
There's a clear drop-off in the number of votes cast.