McCain is hitting Obama for opposing designating Iran's Revolutionary Guard as a terrorist organization.
In fact, Obama had sponsored a piece of legislation in the Senate to do just that. But the bill was going absolutely nowhere. Why? Senators Kyl and Lieberman had competing legislation that not only did what Obama wanted, but also declared that it was vital to national security that the Revolutionary Guard be stopped from building a Hezbollah equivalent in Iraq.
Before the vote, though, those latter portions of the legislation were stripped out. The bill was, in fact, pretty close to what Obama wanted. He opposed it nonetheless, but he didn't vote on the matter.
Hillary Clinton supported it.