When Romney/Obama loses, who becomes the standard-bearer of their party?

Inevitably (and sadly), only one of these two men will lose the upcoming election. So when that happens, what happens to the leadership of their party? If Obama loses, who do you think will rise to take his place as the ideological and rhetorical leader of the Democratic Party? Who will emerge as the major contenders for the 2016 Presidential Campaign? Same goes for Romney: who do you think will take his place? Will someone take his place at all?

I ask this question because it seems like both parties are running out of obvious "stars." Well before 2008, we knew that Clinton, Obama, McCain, and Romney would be contenders. They were either apparent figures or rising stars within their respective parties, or in our political sphere in general. That doesn't seem to be the case any more, it seems like our present cadre of national-level politicians is exhausting itself. I don't see Hillary running in 2016, or Romney again. And the ones who are favorites within their own parties, like Ryan and Warren, just don't seem to be of presidential caliber.

Will new and unknown figures emerge from the fog and rise to the occasion? Or will we be condemned to a slew of mediocre or laughable candidates from both sides after this election cycle? Some historical context from those of you in the know about this stuff is most welcome.