re: The reason republicans constantly go against their voters is because they live in DC.

It's not just professional pressure on The Hill from democrats, it's also social and cultural pressure. If you want to get invited to the best parties and functions -- which are all hosted by liberals as a function of 99% of the city's residents being democrats -- you also have to gravitate toward their groupthink and positions. This is also why conservative supreme court justices move left once they take the bench in D.C. (see, e.g., David Souter). That is always a one-way street. When is the last time a justice who was on the left moved right once taking the bench?

It's not unlike living in Hollywood or Manhattan. If you are a Trump supporter you are not going to get invited to the lavish parties with Tom Hanks and Bono.