Quite a few conservatives have been planning a second civil war anyway due to demographic anxiety about whites no longer having an absolute popular majority, and their goal is to instead establish a majority white ethnostate founded on hardline religious principles. I'm not being hyperbolic here, and will happily supply you with abundant academically rigorous documentation of this. It's not a fringe movement either, but one of national scope and ambition that has been following a gameplan first developed in the 1980s.

Now, I don't expect the western states to secede any time soon, but by 'soon' I mean within the next 5 years. A breakup of the US is fairly unlikely, but large-scale civil unrest is a significant and real possibility.

There is a distressingly large portion of the country that would love to see California leave the us. (Or Kansas or Texas or New York).

Wouldn't a constitutional amendment resolve this?

Sure, but if the point of separation is to resolve a crisis where two sides can't agree I don't see how you'd get them to pass the one piece of legislation that requires the ultra-highest bar of consensus, across both the state and federal levels of government.

A group leaving the country doesn't give 2 shits about the constitution. It comes down to how much you are willing to fight for it, and both sides would end up with nukes.