I'm not sure, of course, but it seems that something happens to people that go into politics. I think it is in large part that there is an inner circle mentality that one cannot escape if one hopes for success. Look at McCain. For years I considered him among the best senators in this nation. I was envious of people living in AZ who had him as their representative. In 2000, for reasons passing understanding, the Republicans anointed a mouth-breather legacy to run for president in liu of a man who was nearly universally respected (GOPers, Dem's, Indi's...). Now, with the 2008 election in view, and many people wanting Sen. McCain to run again, he has turned into the very thing he seemed to stand against.
He--a former POW--campaigned heavily for Dubbya in 2004--a war-avoiding reservist who had swift boat attack dogs chewing on his opponent--a decorated Veitnam vet. He told Americans that they would not be willing to work as hard as immigrants for $20/hr (which is a pretty hefty amount, considering many American citizens work for far less) and rebuffed an audience member who dared to disagree. He suggested at a straw poll that republicans cast votes for Dubbya in 2008 as a show of support. I'm not sure if party leaders have something on him or if he wants so badly to get the party nod that he has decided that prostituting his views is acceptable or if he really believes this crap. In the end republicans still won't vote for him, and now dems won't either.
In the house the black caucus is going pit bull on Pelosi for asking for the resignation of a bribe accepting, corrupt politician, whose defiant stance only makes voters more irate. It has nothing to do with him being black, just like it had nothing to do with Cunningham being white, or DeLay, or any of those that Abramoff bedded with money. It has to do with them being crooks. They need to be gone. There are still good congressmen and women (Sen Feingold jumps to mind), but too many are corrupt, and most have done nothing illegal.
Sen McCain has (most likely) done nothing against the law, but he has become corrupt in his values and his public interactions. The same with the members of the congressional black caucus, most of the leadership (Dem and GOP), and countless others at all levels in all branches. Corruption for a politician is not restricted to impropriety (or the appearance thereof) but is when the office, the positions held, and the perceived importance of self supercede the needs and desires of the constituents who put them there, whom they claim to represent, and who are America. We need Americans to remember that talking about values is not the same as having them, and those haves are the ones we need in office.