The modern American presidency tends to swing back and forth between the two major parties, two terms at a time. Since Bill Clinton defeated President George H.W. Bush in 1992, that’s been the case.
It’s always worth it to win a major-party nomination, even if one is running against an incumbent or trying to succeed a two-termer from one's own party. But when a two-term president is getting ready to retire, the nomination of the opposing party is especially valuable. Whoever gets the GOP nod in 2016 has an excellent chance of becoming president.
