A small but non-trivial proportion of the population have followed that trajectory since 9/11 -- turning into the sort of liberals who think torture is fine and that human rights should be spread primarily through waging preventative war against America's geopolitical rivals. Why shouldn't one of the people to follow that path be a United States Senator?
I'm really not sure I buy the "Joe got all pouty when no one wanted him to be president" theory. He was pretty wankerific before the presidential primary campaign heated up (and, of course, during it.)