George Will says he could, but I don't buy it.
First off, I suspect most Ron Paul supporters aren't really Republicans, but people who are generally disenchanted with the two-party system and fiercely oppose the war in Iraq, the international monetary system, immigration, desegregation, and non-Christian manipulators of Christianity. These people weren't very likely to vote for McCain in the first place. In fact, I tend to think they would have held their noses and voted for the antiwar candidate, Obama, but will now have another outlet for their fundraising and organizing energy.
Second, mainstream libertarians have been pushed pretty far left over the last eight years. They are outraged at what they perceive as infringements on their civil liberties by the Bush administration, and I think many would have voted Obama, and still will, if only to see the Republican party taught a lesson. If one looks at the direction Reason magazine has gone over the last few years, it's pretty clear that McCain is not a big draw for their readers.
Third, Bob Barr is a Change machine--a real turnover the checkerboard kind of guy. The McCain camp can't be counting on support from people who want to overturn the international order as we know it.
So I suppose it's possible Barr might siphon enough Republican votes to hurt McCain somewhere, but if you have a four-way race between McCain, Obama, Nader, and Barr, then you have a race with three antiwar candidates and one all-American hawk. I don't think its particularly counterintuitive to view Barr as a greater threat to Obama than he is to McCain--think Nader in a Corvair.
Of course we all know he's going to be a complete non-factor anyway. Sorry libertarians. :(
