Ronald E. Riggio, blogging at Psychology Today, has a simple explanation:
So, the question is why do politicians so often lie, exaggerate, and distort the facts? The short answer is that it works! People are notoriously bad at detecting when others are lying, and there are several psychological reasons for this.
He goes on to explore very briefly those psychological reasons.
If you pay attention to politics–or maybe especially if you do not pay attention until an election nears–this should be required reading.