It's true that politicians sometimes lie. It's true, in fact, that they may even lie more than most people in public -- though, probably, only because most people don't have the opportunity to talk in public quite as often. But nearly regardless of politicians' actual adherence to the truth, the idea that politicians tend to lie is a serious cultural problem. The problem is that the belief that politicians generally lie makes it nearly impossible to make informed decisions about politicians. It affects both sides: whatever is important to you, you can assume that the candidate you support doesn't really mean it when she contradicts your views, and the when that candidate's main opponent says something you support, you can assume he's concealing his actual motives.
There are a number of other important possibilities one should consider in the first place: That the politicians are honestly repeating a lie that they have been told, that they genuinely don't understand the issue, or, most importantly, that they actually believe the things they say, and intend the things they say they intend.
I don't want to make this a partisan argument, but I want to write an example. So I'm going to make up some politicians, and use policy debates that don't exist at any significant level in America.
Ann is running for the Moose Party. She claims to support adopting the Alternative Vote method for electing candidates, because she thinks it's more fair. She also publicly supports the repeal of the Lollipop Act, which subsidizes the presence of lollipops for children in doctors' offices and banks, because she thinks it's wasteful.
Fred is running for the Polar Bear Party. His platform is that the Alternative Vote is a bad idea, based on the argument that it will decrease voter turnout and increase errors in voting due to confusion. He publicly supports the Lollipop Act, and promises that if he's elected he will block attempts to repeal it.
Now, I support the Alternative Vote, but I also support the Lollipop Act. Of the two issues, I think the Lollipop Act is much more important. I've voted for the Moose Party in the last three elections, though.
Conveniently, though, I know that politicians lie all the time. So I know that the politician I identify with probably believes the same things as me. Besides, voting for the Moose Party has always gone well for me in the past.
So it feels safe to assume that Ann is lying when she says she's against the Lollipop Act. Sure, some fringe elements in my party believe that, and she has to pander a little bit. That's just politics. But she'll never actually do anything to pursue that campaign promise.
Besides, Polar Bear candidates are all the same. Their political philosophy is wrong, so nothing about it is likely to be reasonable. The things they say that I agree with are inconsequential at best, but more likely outright dishonest.
The dishonest politician mythology that pervades American political attitudes degrades the quality of our discourse, and undermines the possibility of an informed electorate. You can't inform a group of people who discount whatever evidence they dislike.
Of course, we do sometimes need to discount evidence. But there are ways to do so that don't automatically reinforce your initial views. Relying on credible sources is a good one, although the credibility of mainstream news isn't what it used to be (if it ever was what it's supposed to be.)
But in most cases, you can generally take a politician at their word. They probably aren't lying as much as you wish they were.