Anybody with a serious interest in protecting America's democratic institutions should pay heed to this article by a former Republican staffer who left his party in disgust. It paints a truly terrifying picture of where we are headed.
Democrats and Republicans would be nothing without each other, they both fulfil a fundamental role in machine politics revolving around controlling opposition.
1: Both parties try to appease various groups, of which rational voters are a small proportion and corporatists are a significant proportion.
2: Both parties oppose whatever the other party supports in order to attract voters, it's like "the price is right", their position on an issue only needs to be slightly more in one direction than their opponent in order to guarantee them support and since both parties realize this they usually compete and get around 50%.
3: Both parties also realize a 3rd party would disrupt this, they don't want to be stuck between 2 prices like some contestants on the price is right, so they cooperate to neutralize 3rd parties.
4: Both parties are a major influence on political culture, if this system results in minor contradictions they are in a position to change people's perceptions of this.
5: It would be no use if one party supported everything the corporations want and another opposed everything, it would be too obvious, so both parties oppose and support corporatists in various ways while harboring the opposite position of the other party as explained. Naturally the parties merely tolerate their token anti-corporatist positions and create propoganda to make it seem as though they care while really putting most of their effort into their pro-corporatist positions.
I can't be bothered to explain everything but I don't think this is really a case of which is the lesser evil, saying Republicans are evil is like saying it was the gun not the bullet that killed JFK.