They'll go out to vote once the GOP spin machine makes it clear that Obama's election will be the end of America/end of the world (again) as he unleashes Satan (literally) to do his Kenyan, Muslim, over-regulating, tax-increasing, gun-restricting agenda.
Gingrich will probably fare a bit better in a general election because he's not truly a hard-right conservative like he's forced to pretend to be in the nomination, plus he's got some name recognition with people even though they forget how much of a screw-up he really was. Remember: people who don't pay as much attention will be a key voting bloc to try to win over.Paul does consistently better against Obama than Gingrich and Santorum. Especially Gingrich does piss-poor.
Paul will probably wilt in support once people actually hear his policies and positions beyond "let the states decide", "get out of the wars", and "legalize marijuana". The details of his agenda has mostly flown under the radar so far since he's never been a frontrunner, but he won't be able to get away with that in the general election.