This is pretty correct. Most local politicians that i know personally are intelligent and highly principled.

When it comes to advancing an agenda (say putting more of the budget into transit rather than road widening) they rely on their staff to tell them how far they can push it without getting thrown out of office, or what aspects of issues are really important to voters so compromises can be struck etc.

At the federal level its probably all too easy to become divorced from reality.

Presumably at higher levels the game becomes much harder as well, so to survive one has to throw those principles away (if one had them to begin with).