That would be Mark Salter, McCain's longtime aide who's best known as his co-author on books like Faith of My Fathers and Worth the Fighting For. In the current campaign, one of Salter's jobs seems to be to sit on the back of the Straight Talk Express and make sure that reporters don't bait McCain into saying something nasty about Mitt Romney--whom it's pretty clear McCain can't stand. The way Salter does this is, when it looks like reporters might be about to goad McCain into talking smack about Romney, Salter will launch into his own anti-Romney diatribe.
I thought this sort of thing was mostly confined to the back of the bus, but I see that, in response to Romney's new attacks on McCain in New Hampshire, the McCain campaign released a smack-talking statement from Salter that reads:
"Welcome to Mitt Romney's bizarro world, in which everyone is guilty of his sins. He didn't support Ronald Reagan. He didn't support President Bush's tax cuts. He raised taxes in Massachusetts by $700 million. He knows John McCain is gaining on him so he does what any small varmint gun totin,' civil rights marching, NRA endorsed fantasy candidate would do: he questions someone else's credibility. New Hampshire is on to you, Mitt. Give it a rest. It's Christmas."
You almost get the sense Salter would have rather said (go to to the 4:30 mark; NSFW):