You and parent poster are both wrong. A politician rarely ever lies. Incredibly, most career politicians sincerely believe what they say. They just change their beliefs more often than most people, even several times a day.

Whenever some specific belief is challenged by external conditions (poor polling, disagreement with wealthy donors, etc), they just go through long doublethink processes multiple times with their advisors, until they're persuaded that changing their position is the best thing to do. Once they believe what they say, they simply sell it like only Real Believers could.