"Mr. President, when I voted to give you the authority to go to war I honored you, as millions of Americans honored you, with our trust. We trusted you to check and re-check your evidence before you committed us to war. We trusted you to exhaust all diplomatic avenues before you committed us to war. We trusted you to have a detailed, organized, flexible, effective plan for the post-war before you even considered committing us to war. Sir, you did none of these well, and I and a majority of Americans feel betrayed by your performance. You didn't listen to dissenting voices in the CIA about the evidence. You didn't listen to generals, long-time allies, fellow Republicans-anybody who didn't already agree with you-even though they had legitimate concerns that would have improved planning. You didn't listen to anybody who suggested that our troops might not be met with flowers. You committed us to war in such haste that you sent our troops on their way without adequate armor. The buck stops with you, sir. Americans trusted you with the decision about going to war; I trusted you with my vote. Now you call us flip-floppers because we hate how you misused our trust. Sir, have you ever acknowledged to the American people that you made a mistake, any mistake, in taking us to war the way to did?"
Addressing this to his face, talking to him (no matter who asks the original question), delivers a powerful non-verbal signal of strength-that Kerry has the cojones to speak truth to power. It also has a good chance of triggering a reaction in Bush that will finish him as "leader." Doing this earlier, in stump speeches, doesn't leverage the direct authentic power (not to mention the audience) of the face-to-face confrontation and it allows Bush and the evil spin conglomerate to test all kinds of responses and lessen the blow. By saving it for the debates, he gets to have Bush respond without any benefit of spinmeister advice and he gets to confound expectation positively by laying a groundwork of "nuance" and "waffling" only to destroy it suddenly in the debates.