They should pass the legislation necessary to do this with a condition that it only comes into effect the moment any republican state changes their boundaries. Every democratic state should do similar.

This prevents the scenario where they try to sneak it in without time for reaction at the last possible second.

It also massively disincentivises making any changes whatsoever

Also maintains some level of moral high ground (which seems to be an issue for some people as the country is stolen from beneath them) as it is only triggered self defensively