Begin with the McCain quote from May, where he says he won't take the low road that it's not necessary, etc. etc.
Segue to a list of quotes from "respected" sources such as Joe Klein saying, "This is despicable." "McCain is not an honorable man," "not running an honorable campaign." Show clips from the ads McCain has been running lately.
End with the voice-over: "With McCain surrendering his honor in order to win the election, what is there left to vote for?"
Fade to picture of him embracing Bush.
Okay, this is rough and quick, but the point is, go after McCain's strong point, his supposedly "honor." Yes, make the accusation that he's surrending his honor--that's the red meat that will make the media go nuts. But the accusation has the distinction of being (a) the truth, (b) something MANY other people are already saying.
Bonus: it will probably cause McCain himself to lose his temper...and you won't like McCain when he's angry.