I will ask the question I have not seen asked.
Does it matter that Moody's downgraded the US to Aa1?
After all they are the third of three major rating agencies to do this, and the first one did it 14 years ago. And it is not like it affects anyone's ability to hold US government debt. And with the ability to print money it is not like a default is likely either (though for brinksmanship reasons it is sometimes threatened).
So what does it really matter? If your answer is that it will make the cost of borrowing higher, that isn't true either. In 2011 interest rates on treasuries went down after S&P downgraded. If you are a corporate issuer it would be true, but the official rating of US government debt does not have any meaningful impact on demand for the securities, so rates are not affected by the rating.
For sovereign credits ratings are mostly a trailing indicator. Moody's is just confirming what we all already know. Debt is too high, and the latest budget proposal only adds to the problem.