No, actually, it’s not. Practically all the coastal counties of North Carolina are red, for instance. Get away from the oceanfront counties and they’re practically all red, military bases. There are red coastal counties in, get this... California.
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.