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.
After two years of Worst Trump (or even Pretty Bad Trump) this could be far closer than anyone thinks right now. Especially if he's fighting with Silicon Valley. A hypothetical liberal-libertarian anti-Trump alliance could have real legs in CA.
Every time the Dems lose an election, Californian independence gets brought up. Every time Republicans lose an election, Texan independence gets brought up. The other 48 states wish California and Texas would get over themselves.