Suppose a constitutional amendment were proposed that would allow a state state to secede from the union if it was supported in a referendum by 60% of the state’s population.
Should progressives support this?
Well, yes.
Splitting up the US is a BAD idea, but I don't see how you should go against the wishes of 60%+ of the population of a state.
However, there should be a transitional phase where people could move in and out of the US and that state before it took effect. We wouldn't want US citizens suddenly trapped in some hellhole Qanon regressive state with no way out.