THis one is damn brilliant.
It's here.
What's so great about it?
Because it directly confronts the two questions that have hurt his campaign - (1) is he too young? and (2) can he really win.
It shows him running through an Iowa town, intercut with voters reacting to him, while he says he's been underestimated all his life and has always won.
It's the best ad I've seen outside the MoveOn contest.