Whatever one thinks of the credit-rating agencies—and we aren't admirers—it serves no good purpose to shoot the fiscal messengers. Friday's downgrade by Standard & Poor's of U.S. long-term debt to AA+ from AAA will be the first of many such humiliations if Washington doesn't change its economic and fiscal policies.

Investors and markets—not any single company's rating—are the ultimate judge of a nation's creditworthiness. And after their performance in fanning the credit and mortgage-security mania of the last decade, S&P,...