Why is it when we FINALLY have a centrist candidate, we're all pulling toward the partisan one..?

I mean, Obama seems to mean well, but he hasn't been able to stand up to the extreme left in his party, EVER, and, as such, has wracked up one of the most liberal voting records in Congress. And, have you listened to Nancy Pelosi sharpening the socialist knife..? Why isn't Obama distancing himself from that, the same way McCain has put down the "arab" and "muslim" crap that's come from the extreme right..?