However, I think that Mitt Romney hit the nail on the head. From a post on this blog in 2007:
"I just came back from living in a state that was overwhelmingly Republican," Romney told the [Commonwealth] magazine [in July 2002]. "And a single-party system generates problems, in my view. In that state, the more extreme wing of the dominant party begins to have more and more political sway, and that de-energizes the great mainstream of citizens. They get turned off by politics and offended by the politicians and become less engaged in the political process."
Of course, in 2002, Romney was running to be the Republican Governor of an overwhelmingly Democratic state.
Hmmm, sounds familiar.