From Politico, a week ago:
Paul told Politico recently that he has “no intention of running as a third-party candidate” if he doesn’t win the GOP nomination. (He was the Libertarian Party candidate for president in 1988.) Before an appearance at American University in Washington, Paul said he would endorse the candidate who “promises to get us out of Iraq and support liberty at home.” (At the moment, no other Republican candidate has signed up for a withdrawal from Iraq.)
But of course it's hard to imagine either major-party candidate promising to get us out of Iraq, or endorsing Paul's definition of "liberty at home." Which in turn clears the way for Paul to reluctantly endorse ... Ron Paul!
Update: However ...
There is a potential stumbling block to Paul jumping to the Libertarian ticket: A number of states, including Paul's home state of Texas, have "sore loser laws" that prohibit a candidate who loses in the primary of one party from appearing on the ballot with a different party in the general election. "If Paul were to seek our nomination, he'd have to make sure his name wasn't on those ballots," said Cory. "It would not disqualify him from being our candidate, but would weigh heavily on the minds of our delegates."