Ron Paul talked to Fox Business about where the presidential vote of his fans might go:
Paul said, "Some will be angry at the Republicans for the way they retreated at the convention and they might not show up. Some may vote libertarian, some may go with Romney, and actually, some of the young people because of foreign policy, may even go with Obama. […] I know one thing for certain, that they're not all going to the one place because they're very individualistic and they don't see the consequences exactly the same."
For the former candidate, it goes back to frustration over the two-party system and why there are only two candidates who are debating tonight. Paul has not endorsed Romney, and in fact, he says he hasn't decided who he is going to vote for. Though, he added, "I don't think anybody thinks I'm going to vote for Obama."
Gary Johnson of the Libertarian Party would certainly like Paul's fans, and Paul's endorsement, though I suspect that the obvious choice on the part of the larger Paul political machine to infiltrate and shift the Republican Party makes such an endorsement unlikely. Paul has, though, said he finds Johnson "wonderful."
For the full story of the political career of Ron Paul and his supporters, see my book Ron Paul's Revolution.