It is well-known that John McCain has some problems among the more conservative base of the Republican Party. Many right-wingers don't trust McCain and don't really consider him to be one of them, since he has deserted them on some issues like immigration. But he is the nominee, so it was expected that they would hold their noses and vote for him anyway.
But something happened yesterday that may change that. Former-Rep. Bob Barr announced that he is seeking the nomination of the Libertarian Party to be their presidential candidate. Barr is a person that has much more credible conservative credentials than McCain.
All of a sudden it looks like those unhappy with McCain now have an alternative they can vote for. It is entirely plausible that Barr will be a spoiler for the Republican Party -- much like Nader has been for the Democrats in years past.
Barr still must win the Libertarian nomination, but I think he'll be able to do that. They really don't have anyone else running that has his name-recognition.
This makes things look even better for Obama in November.