Now I don't buy into this. I think quality people can and do get into politics, despite the two party system. I have dealt with Carl Levin a couple times in my life, and while I don't agree with him on everything, he strikes me as very principled, and at time willing to buck the party(sometimes wrongly, see the 2008 democratic primary).
However, I think the reason why there is such a high number of scumbags in politics and other nationally exposed jobs(pro sports athletes, movie stars) is because to be willing to do what they do requires a very pointed drive which does not allow a lot of room for other things.
I remember an awesome interview with Barbara Bush shortly after her son took office, and the topic got around to the Clinton's, and she was asked to say something nice about them. Without hesitation she said(paraphrase, my memory not that good): "I could say lots good about them, but the thing that stands out in my mind is how they insulated Chelsea from the press, and did all in their power to give her as normal a life as possible. That is so hard to do when you are in Washington." The thing that stands out was the "as normal as possible". if you have kids, to be in Washington means your kids won't have a normal life, and you have to be willing to sacrifice that. I don't think most good people will do that.
As an aside, if you can, read The Final Days by Woodward and Bernstien. Nixon's daughters and their husbands are discussed in some measure in that, and I felt strongly for them and what they went through the last year of Nixon's presidency.