Trump would impose a tariff on all goods coming in. And I would hope th Us govt would shut down all military and US govt aid and jobs in California if so, regardless of political party
Tariffs would hurt US more than it would California. Tariff is a cost paid by US citizens to benefit non-competitive US suppliers.
And assuming it happened, how would California protect its borders from its neighbor US?
No, that way lies madness.
California needn't have heavy border patrol if US-California relations are peaceful. For reference: see European borders. You can just walk over to another country most of the time.