One whose own party says "don't take him seriously"?
Exactly. "They don't take me seriously, so I'll show them. I'll run as an independent and ensure that Obama gets re-elected. That'll teach them to mess with The Donald!"
If he can't win the nomination in the party that's the home of most conservatives, then how does he expect to win the general election? And oh, suppose the nominee was Palin, or Bachmann. Would you still be thrilled if he ran as an independent?