Repucrats and Demoblicans like to be distinguished from each other, by proposing a little more or less state intervention here, a few subsidies more or less there, a few taxes more or less yonder, switching roles on various topics. This way, they get people to follow either party depending on their various preferences. But all in all, they play the same game as any other pair of bullies: Good Cop, Bad Cop.
Each voter quickly caves in and accepts one party as his savior from the other party, and signs a blank check to the State as its sovereign. And in the end, citizens have to do all the things that these two parties agree upon. Political Power is a bi-partisan issue.