James pretty much nailed it, but i think manipulation plays a bigger role.
i dont mean to sound like a conspiracy theorist, but to me the reason why there is so much influence on taking one side and bashing the other is because both partys use it to gain support. if there wasnt another party to blame for the wars and recessions, we would have to look at the people we are following. somehow the people must be convinced that the government is not at fault. we have a clever tactic in America where the people say "it is the side of government I dont support that is making me unhappy", rather than" the government makes me unhappy." when you think about it there is a big difference. they are still supporting part of the government this way, and no ones going to try to severely change a government they agree with, even if they only agree with half(or less) of the things they do.
These partys can bash and tear at each other as much as possible, get people to point fingers at the other side, blame it all on far left or right ideology and even certain people in particular like This idiot Glen(who has done very well in life for an idiot i painfully concede), but all this bashing is just a way of showing your support for the fraction of the government you pick.
to sum up my conspiracy theory:
Hate for [size="3"][color="#ff0000"]R[/color][/size]=support for [size="3"][color="#0000ff"]D
[/color][/size]Hate for [size="3"][color="#0000ff"]D[/color][/size]=support for [size="3"][color="#ff0000"]R[/color][/size]
[size="3"][color="#ff0000"]R[color="#000000"] +[/color][/color][/size] [size="3"][color="#0000ff"]D[color="#000000"]= [size="2"]government.[/size][/color][/color][/size][size="2"]
government= hate = support
people hate, yet support their government in this system.
[/size]