UPDATE: OK a little more said. I think this is a pretty good point from Mark Halperin, via TPM:
On CNN just now, Mark Halperin pointed out that one reason this is a big blow to McCain is that the press will talk about the endorsement for the next few days, cutting into the time McCain has left.
This took me back to McCain's selection of Palin as a way of stepping on Obama's convention bounce. But all bounces fade, so they were basically stepping on an illusion. Obama is stepping on McCain's lifeblood here--time. That isn't an illusion. He just robbed McCain of oxygen. They are slowly tighnening the grip. The cobra-clutch is in full-effect.
UPDATE #2: Video below. That Muslim answer was incredibly, incredibly, incredibly strong. Stronger than anything Obama's ever said. That was just beautiful. That part should be everywhere. It was really moving. So what if he is, indeed. More later, I need to grapple with that answer. It was heavy. I imagine a lot of Muslim cats (I'm thinking Keith Ellison) are like, "Finally."