There’s no way to get every last person, but so long as a solid 2/3 of the coast was liberal and the middle conservative, that’d be enough to guarantee political dominance for the foreseeable future. There are one or two coastal counties in California that might be red on a good day, but barely anyone lives there.
You must not be in a state you’re proposing to lop the coastline off of, sounds like some sort of Texas thing. Every large city in Texas is liberal. You’d be keeping Chicago, Detroit, Minneapolis, St. Louis. And giving up how many ports and military installations? It’s a pipe dream and not a very smart one.