Uh... even in California, this would lose 70%-30%. It might even provide a rally round the flag effect for Trump in 2020 and get him to 40% in the state.
Nothing would get Trump 40% of the vote in California. Just because people might vote against a secession effort doesn't mean they suddenly become Trump fanatics.
One thing that got completely ignored during this election is the fact that there was no GOP senate candidate running in California. Unfortunately, they adopted that nonsensical LA system where the top two vote getters are put on the general ballot.
Imagine how many republicans stayed home because they knew there was absolutely no point in showing up. Trump wasn't going to win the state, and they had no Republican candidate to vote for in the senate. So why bother?
I guarantee that this cost Trump at least 5 percentage points.
Why is it nonsensical? A Sanchez/Harris was more competitive than any race between Harris and a Republican. If the real race is the Democratic primary why not move it November when more people vote and also give the opportunity to everyone, including non-Democrats to vote?