This occurred to me today while reading
this article by Politico. S&P, Fitch, and Moody's are all American rating agencies, yes? If push comes to shove, the easiest way to stave off the threat of a downgrade would be nationalize all three companies. Obviously that's politically impossible, but we all know there's no way a national credit rating agency would play hardball with itself. Of course it would lose all credibility with the markets, but in the interim until a new rating agency is established, the threat of a downgrade would effectively vanish.
The same thing could probably be accomplished by declaring those three companies trusts and busting them up; however, considering there are three of them, that would smell too fishy. Besides, that wouldn't give the government as much time before a new, reputable company surfaced to downgrade them.