The argument about whether Bob Barr would take more votes from Obama or from McCain is an interesting academic exercise, and I agree with Noah Millman that one can imagine all sorts of interesting ways that a Barr bid could affect the election. But even if Barr gets the Libertarian nomination - by no means a sure thing - I'm distinctly unpersuaded that he'll get enough media coverage or raise enough money to be more than a very, very minor factor in November. I have a few reasons for thinking this, but the biggest one is that nobody likes Bob Barr.
Well, fine, "nobody" is a little strong - the guy won four House elections, after all. But Barr is neither a political icon for a generation of true believers (see Nader, Ralph) nor a natural rallying point for a resentful identity politics (see Thurmond, Strom and Wallace, George) nor a massively wealthy, massively entertaining, essentially sui generis figure like Ross Perot. If there's a promising parallel for Barr in the annals of third-party politics, the closest one would be John Anderson, who did manage to swipe seven percent of the vote in 1980. But for Barr to run as a Libertarian in 2008 isn't really like Anderson's Rockefeller-Republican campaign in '80; it's more like if Anderson had left Congress in the early '80s and then changed his ideological colors sufficiently to mount an independent bid from George H.W. Bush's right in 1988. Okay, that's an imperfect analogy, but hopefully you take my point: There's space for a spoiler candidate in this race, but Barr isn't the right person to fill it. He's too uncharismatic and too unknown, and to the extent he is known it isn't for the sort of politics he's lately adopted - rather, it's for impeaching Bill Clinton, which doesn't seem like a particularly useful calling card in the '08 election.
My bold prediction: Barr will do roughly as well as Pat Buchanan did in 2000, benefiting from the more favorable landscape for a right-of-center protest candidate but suffering from a lack of the charisma, celebrity and committed followers that Pitchfork Pat brought to the '00 race. He won't be included in the debates, the media will largely ignore him, and he'll end up being a factor only if the election comes down to some butterfly ballots in South Florida. There is an anti-war, anti-immigration libertarian who could have had at least a Nader-style impact on the general election - but his name is Ron Paul, and he isn't running.