Serious answer? Someone who's actually fairly left of centre (as opposed to the US Republican definition of left-wing, which sees Barrack Obama as some kind of Marxist). I can't think of anyone specific (although please not Santorum!). But quite frankly, I'm hoping for another four years of Obama. His Presidency may not have been as radical as it could have been, but after eight years of Bush Jr, anything would have been an improvement.
Non-serious answer? I know he's not a US citizen, but can we have the Doctor as President of the USA forever? That would be cool.