For politicians to lie is acceptable because there are no consequences and because it doesn't change the available choices. In the late stages of a political race with both sides lying it's not like a third choice becomes available. Not to mention not everyone will learn every truth behind the lie so a good percent of the lies go unchallenged. The ones that don't not everyone sees the challenge. Politics is a game of percentages and we have focus grouping down to a science. A lie designed to sway the opinion of an interest group that isn't likely to look up the truth is very effective whether or not it's challenged. Tell enough targeted lies and you win.