He should reiterate his commitment to refusing money from lobbyists and special interest PACS, emphasize that the DNC is following his lead and challenge the Republicans to do the same. Then he should say that the whole point of the public financing legislation was to limit the influence of just such donors, so he is being true to the spirit of that legislation. But not, he should add, right up front--to the letter of his original commitment, a commitment he made way back in March of--2007.
He should dwell on that date. March, 2007. Then he should say something like:
A lot has happened since then, as you know. Circumstances have changed and, when that happens, policy should sometimes change as well. Nobody dreamed that our campaign would get that outpouring of support--X million in donations of $250 dollars or less from Y millions of donors who aren't affiliated with any corporate special interest or lobbyists. That's more than Z times the number of any other candidacy in history. If I were to ignore this change in circumstances, if I were turn my back on the ordinary Americans who want to support our campaign in this way and, yes, if I were to relinquish the advantage that kind of support gives our candidacy in this watershed election--well, I really would be naĂŻve.