Pretty much all the Republicans will vote for him even if they don't like him because he's 1) he's a Republican and 2) he's preferable to Hillary.
Remember 2006? I don't think that's a given.
Contrast this to Thompson, who will get all the conservatives, lose all the liberals, the libertarians will vote third party, and Hillary gets most of the independents.
More than half of poll respondents say they "won't vote for Hillary ever," or something of the sort. Also not a given.
And Thompson has a much, much better ability to communicate in written and spoken form, and on TV, than any other candidate in the field, bar none. That won elections and hero/hated status for JFK, Ronald Reagan and Bill Clinton. Not Hillary, Bill. Hillary doesn't have "it", and Obama doesn't either, "clean" though he may be.
Libertarians may or may not vote Lib. Paul didn't get many votes in '88, and Badnarik got a smaller percentage than that in '04 AFAIK. Again, not a given. The Libertarian Party lost a LOT of members in 2002, and many of them have been part of the blog revolution since then. Many left over Badnarik and his anti-war dogmatic stance.
Hence, we read and hear from a lot of independent libertarians, not Libertarians, and the LP lost its only real remaining voice when Harry Browne died, may he rest in peace. Miss him.
Tech Central Station probably has more regular readers than there are LP-registered voters in the US, for example. Ditto for Instapundit and Reason.
If Paul runs, it will probably be as a third-party candidate (with 10 current GOP candidates, and at least 9, maybe all 10 of them have to lose). Of course, that worked for Joe Lieberman, but he's sure not on the libertarian right!
Anyway, I don't see any "givens" other than that each candidate has different qualities, and that some of them will be found to lack those that get someone elected. Exactly what and whom, we won't know until after the fact, and then we won't always REALLY know more than what professional guessers write.
Thompson has speaking abilities not seen since Reagan, and he has more of them than Reagan. He can write, too.
In many ways, he's the Conservative standard-bearer that Bush has failed to be. Liberals and committed Democrats hated Bush from day 1. Bush's drop in approval has more to do with those who DID approve of him at first: conservative-leaners and Republicans.
But this much is fair to guess: Paul won't get one bit of the liberal vote. The whole liberal thrust right now is towards domestic policy of raising taxes, spending more and regulating more. If he makes it to the General Election, he'll be debating about THAT, not the war, and it won't get him a single liberal vote.
So I don't think that there are too many "givens". I suppose that's good, since the current "front runners" on each side all suck IMO.