If Romney wins, he will be the nominee in 2016. Palin (or whoever else) will have to wait until 2020. For what it’s worth, Palin will still only be 56 years old in 2020 - seven years younger than Romney is today. So Palin has plenty of time. But based on my observations of her over the past year, I feel that she may be out of politics for good.
Romney managed to be the last one standing in the candidacy, but we could do (much) better with Palin.
Palin changed her mind. Her kids will have grown up greatly in the next four years, and she will be ready.
Romney in 2012.
Palin in 2016.