Why Is This The Case?

Here is a state-level chart of political beliefs in Congress. It makes sense that some states tilt more toward the right and others more toward the left.  However, Eric Crampton asks why some states have significant overlap (North Carolina, New Mexico, Virginia, Ohio) whereas others do not.  New Mexico, Virginia, and Ohio are generally battleground states in Presidential elections as well, but other states which are usually considered close—Iowa, Wisconsin, and Pennsylvania—don’t have any overlap.