It's simple reciprocation from game-theory perspective: when the other party lies and gains advantage, you do it too. But I feel Republicans are way more untruthful than the Democrats and simply bring down the level of discussion for everyone.
And I don't know why is it, that majority of Americans are so accustomed to lying that they tolerate it from politicians. In Finland if politicians get caught with outright lying, it's career-ending if it's bad enough. Prime ministers have resigned over it, party leaders seen their polls slump. In US if Republicans are caught lying (Democrats too I guess to lesser extent), it's a simple distraction and meh from public. Their supporters don't simply care and I guess are not capable of criticizing their own party.
I've been recently thinking that what US has now isn't that far stretch from a single-party system since the parties in power don't really have to fear losing their support. There are no other options, so they can just pass the ball between each other, putting up a show and doing whatever since people can't really choose anything else. If your only option is picking between two assholes, which one you pick? And when the population doesn't know to demand change, what will happen? Nothing.
Yes there are advantages with two-party system, but what I see it has really rotted the political discussion and ideologies into two boxes that can't possibly overlap. In multi-party systems coalition governments can be bit sluggish but at least they offer people a real opportunity for change, and letting go of the old, obsolete power structures. For a country that is so keen on free-market and unrestricted competition, you are awfully restricted in your choice of political parties.