The reason he's lagging against the other Republicans is he doesn't appeal to many people who are registered as Republican. These people are only comparing him to the other candidates - Republicans critiquing Republicans. Only people registered with the Republican party will be participating in the primary elections.
But it's different when the poll's base is not only republicans, but people of any political affiliation. Though Paul is only 3rd when it comes to grabbing votes of Republicans, one thing he does that the other candidates simply don't is grab votes from all other political parties. Libertarian ideas basically take the best of all worlds and combine them into one. Therefore, he has a good portion of Republican votes (those who lean toward conservative, limited government) and he also nets a large portion of Democratic support (from those who value domestic and international freedom and peace). Obviously he'll basically have all the Libertarian vote in the bag, as well as portions from almost all other minor parties.
That is why he isn't first against his Republican running mates, but at the same time he is best when compared directly to Obama.
Basically the only thing stopping Paul from getting the Republican nomination and going into a 50/50 fight with Obama is all the dim-witted, stubborn, angry, nationalistic "Republicans" who are too busy throwing fits over their bogus cable boxes to bother learning any history or economics.
The reason he is a member of the Republican party is for exposure. Independents and minor parties simply don't get any. We have a two party system - if you're going to get anywhere, you have to suck it up and work within the system whether you want to or not.
Gotcha! Fact: You don't know
any libertarians who would not vote for him based on his foreign policy.
A big point about libertarianism is that it is all-encompassing. You can't be libertarian on one thing but not on the other because every little detail is related to every other little detail. If that's the case, then you aren't libertarian but are either republican or democratic.