The way that I think about the DP and RP is that the are not left and right. As Julio said they are both parties of capital and for capital. Therefore, they are both right parties if one understands leftism as anti capitalist. The differences are of course in how they deal, or say they will deal with, the problems that capitalism produces, as they reproduce it. The DP's liberalism will offer more scraps to lower income workers and attempt to 'perfect' capitalism by claiming to be able to remove the social ills. The RP's conservativism will use the appeal of being rich to increase individuality and competition while claiming to be putting the breaks on the social ills caused by the immorality of the market. But make no mistake neither is a party of the left.
The protection racket rests in the ability to depoliticize the economy that arises out of the binary between the two parties programs to deal with the problems created by capitalism. They of course offer no real solutions to the problems, but they sell their protection or solution to the problems. When leftists think and pursue a left program through the DP they buy into that product of a solution and in the process produce the binary necessary to depoliticize the economy and reproduce capitalism. That is the strength of the two party system for capital. It can only be broken by a true independent workers party.
Brad