2020

[Mr. Morden]

It's still early.  We'll have a better idea of potential candidates in about a year's time, when we see what the landscape is for the 2014 midterms, and at least who's likely to be the nominee for each of the major races next year.  However….

Here are some Democrats who might run in 2020 if the incumbent president is a Republican:

Cory Booker
Charlie Crist
Andrew Cuomo
Kirsten Gillibrand
Kamala Harris or Gavin Newsom (in the unlikely event that there's an opening in the CA governorship or senate seat for either of them by 2016)
John Hickenlooper
Tim Kaine
Amy Klobuchar
Lisa Madigan
Martin O'Malley
Brian Schweitzer

Here are some Republicans who might run in 2020 if the incumbent president is a Democrat:

Sam Brownback
Chris Christie
Ted Cruz
Ken Cuccinelli
Nikki Haley
Bobby Jindal
Susana Martinez
Rand Paul
Mike Pence
Marco Rubio
Paul Ryan or Scott Walker

Obviously, much depends on who the nominees are in 2016.  For example, on the Dem. side, if Clinton runs, she likely clears the field, and we so no major challengers in the 2016 primaries, so no Cuomo, O'Malley, Klobuchar, etc. candidacies.  If she then wins the nomination, but loses the general election, then most of those candidates who wanted to run in 2016 would seize their chance in 2020, so you might well end up with someone like Cuomo in 2020.

OTOH, if Clinton *doesn't* run, then many of the above names will run in 2016.  If the 2016 nominee is a man, like a Cuomo or Hickenlooper or O'Malley, then there's a good chance that they pick a woman for VP (Gillibrand and Klobuchar being the most likely possibilities).  If the ticket then loses, there'll be a strong desire to finally put a woman at the *top* of the ticket in 2020, and that losing female VP (be it Gillibrand, Klobuchar, or someone else) will be considered the early frontrunner, at least on the basis of name recognition.

On the GOP side, many of the leading 2016 possibilities are relatively young and haven't run before (e.g., Christie, Jindal, Paul, Rubio, Walker).  If one of them wins the 2016 nomination but loses the general election, then I could see whoever came in second being the early frontrunner for the 2020 nomination.  Isn't working out that way for Santorum in 2016, but I could definitely see the runner up rule returning for 2020.