He can claim the mantle of the true peace candidate - saying that he'll stay in Iraq, keep control, build up the Iraqi army and keep US casualties down. Obama's pullout, he can warn, would mean an inevitable third Iraq war. Obama is stuck seeming either naive - or just as likely to get us into a war as President Bush was.
The success of the surge has created an ideal situation for McCain. What had been the chief Democratic argument against the Republicans can now be their best tool to destroy Obama.