If you are elected into a position where you can enact change, those who elected you have expectations of you based on the policy you supported during the election.

If, then, you turn around and do something completely different your actions no longer reflect the will of those who elected you, and you are not behaving in a representative manner and thus in an undemocratic way.

So ignoring anything specific to the American system, class interests, etc., it is a losing battle to try and be anything different from the status quo and getting elected by aligning yourself with the status quo.

A communist who gets elected by siding with a fascist is no longer a communist. A liberal cannot be a liberal if they denounce capitalism and side with socialists. They are fundamentally different ideas of who the political economy is designed for, completely contradictory ideas about hierarchy, property rights, human rights, and even what constitutes truth (liberal ideas are often utopian, like the "rational economic man", and socialist/communist ideas are often based in the reality of the current and past material conditions, like believing people need homes and food, and a wealthy society should be able to provide these for itself, so people get homes and food. In contrast a liberal society would let the "market" provide these things in whatever way is profitable.