Credit rating agency Standard and Poor's on Monday downgraded the debt of mortgage finance giants Fannie Mae and Freddie Mac. The downgrades to AA+ are part of the continued fallout from the agency's decision to drop U.S. sovereign debt to AA+, an unprecedented move that rattled investors and sparked concern over the long-term impact of the action It was widely expected that S&P's downgrade of U.S. debt would roll downhill to other entities that are closely linked to the federal government.