Heh
For people so dead set against defamation, they don't seem to mind defaming others.
It's not defamation, they did it to Buchanan the last time he ran. Buchanan would actually help Paul in the Primaries, Buchanan did well in 1992 and 1996. The two are close on foreign policy, and limited government, but far apart on free trade (although they both favor getting out of the WTO, NAFTA, etc).
The problem is Buchanan endured an amazing smear campaign in 1996, and the same old BS questions would be brought up again. On top of that, he was scared to death that he gave the election to Gore in 2000.
Out of the current field of Republicans, the best choice would probably be Tancredo, as he wants out of Iraq in the next 6-12 months.
An outsider would probably be best, one with a squeeky clean record. At some point they will try to tarnish Paul as a racist, anti-semite, or sexist since that is the M.O to get rid of a popular candidate.
Someone like Gravel wouldn't work, the last thing pro-Small government voters want is to be one heart attack away from a big government democrat being in office.