Say the words 'Weapons Of Mass Destruction' to any group of people, and see what their reaction is (cue the 'aw jeez, not this shit again' guy). Seven years of 'He lied about WMD!' propaganda came from which candidates side of the argument? The other candidate will be treated to 'more of the same',(this is what that slogan really means, I think) so McCain wins. He cracks down fast, gets the 'he lied' treatment. If anything of a WMD nature happens after that, he's vindicated and then has carte blanche to mop the floor with whoever he thinks is responsible.
They have allowed the Democrats to determine the parameters and to establish the premises of the debate. This whole area is Republican strength - and it has been effectively neutered through Republicans not forcing the issue.