Michael Goldfarb counterintuitively argues that Bob Barr will hurt Obama.
Ron Paul voters would seem to be the irreconcilables of the Republican party. They aren't going to vote McCain no matter what, but they might have voted Obama to punish their party and force a withdrawal from Iraq. If Andrew Sullivan is any indicator, supporting Ron Paul and Barack Obama are not mutually exclusive.
Look, I love Andrew Sullivan, too, but using him as a demographic indicator is like using a black swan to make a point about water fowl. Ron Paul's coalition had three main sectors: anti-war leftists, Bob Taft conservatives, and unreconcilable political outcasts. The anti-war leftists will move over to Obama. You can guess what will happen to the unreconcilables. The Taft types will be extremely gettable for Bob Barr, who will be building a coalition that won't look exactly like Paul's.
Where does a lot Barr's potential base come from? Embittered white conservatives who will not vote for McCain but could never vote for Obama. Some of these people are racists (or people who put too much faith in the e-mails their cousins forward them); most, hopefully, will be talk radio listeners who consider McCain a quisling on immigration, taxes, free speech, etc and etc. They've spent the primaries casting protest votes for Paul or Huckabee, or putting on Operation Chaos fatigues and voting for Clinton. I'm a huge skeptic about the salience of the immigration issue, but with a Republican base this depressed and angry, immigration can be effective wedge for Barr. He doesn't have to finesse his position very much to attack the guy talk radio calls "Juan Amnesty McCain."
The effect this will have on the election, of course, isn't just a zero-sum vote-for-vote effect. It has the potential to box in McCain the way Ralph Nader boxed in Al Gore. How much can McCain brag about his immigration reform cred in public, to Hispanic voters, without Barr rallying the talk radio vote? How much time or money does Barr make McCain waste in a gimme state like Georgia or Alaska, the way Nader made Gore waste time in Oregon and Minnesota? Republicans are avidly hoping that Paul does not rent his donor list to Barr (or Chuck Baldwin) because they see the potential here.
(Obviously, the calculus changes a little if the LP nominates Root, or a left-libertarian like Gravel or Ruwart.)