I'm afraid that Barr wouldn't campaign strongly against McCain. Just imagine (unlikely, still) that Hillary gets the democratic nomination and Barr the libertarian one and then helps her to beat McCain. That is a little hard to believe, isn't it?
The republicans would of course be happy if the Libertarian Party wouldn't pull over Ron Paul supporters. Now, Barr seems to be popular around here, but really, has he any interest in getting the votes?
This is the weird part. He runs on electability, but he's too close to the GOP to honestly have an interest in being elected.
Some people on the right who think like this seem to like the prospect of Gravel cutting into Obama, yet others seem to think that it isn't sane to toy with fire. So he won't be enjoying broad support from there either.
Which means that the right wing of the Libertarian Party might end up not determining the nominee.