I learned this from The West Wing, along with most of my knowledge of how federal politics works. There are conservative Democrats, and liberal Republicans. Being a bleeding heart liberal in a swing state isn't going to win you the a seat.

This is in strong contrast to Westminster parliaments, where political parties are for the most part a monolith, only occasionally allowed a "conscience vote" (that's not to say there isn't infighting). To vote against the party is a very good way to get booted from the party, which as far as I'm aware is practically impossible in Congress, the closest you have is being expelled from a caucus.