Even without a debt default, it looks increasingly possible that the world's credit rating agencies will soon downgrade U.S. debt from the AAA standing it has enjoyed for decades.

A downgrade isn't catastrophic because global financial markets decide the creditworthiness of U.S. securities, not Moody's and Standard & Poor's. The good news is that investors still regard Treasury bonds, which carry the full faith and credit of the U.S. government, as a near zero-risk investment. But a downgrade will...