Politicians care about be re-elected, passing legislation, and accumulating power. The may or may not also care about honesty in their discourse. Say they do. Is it reasonable to expect them to be honest when honesty is not the best route to re-election, the passage of legislation, and/or the accumulation of power? Probably not. The base we can do is understand this brute fact of politics and try to call them out when they lie.
Part of the reason lies and manipulation work so well is because constituents often prefer them to the truth. The more we insist on getting the facts right and keeping the arguments straight, the more politicians will discover that their interest lies in keeping public discourse honest.