En France, les positions politiques sont souvent classées entre "gauche" (Left-wing, progressive) et "droite" (Right-wing, conservative).
So in your case maybe "ville de gauche", ou "ville qui vote à gauche" ? As long as it's in a US context you can use "liberal" I guess but be aware that some French people will understand something else, so maybe say "une ville libérale (dans le sens américain du terme)" ("a liberal city (in the american sense of the word)")