If the U.S. enters a deep recession, a bailout to Fannie Mae or Freddie Mac could threaten the United States' AAA credit rating according to a statement from Standard & Poor's. Government sponsored enterprises (GSEs) such as Fannie Mae and Freddie Mac are large in size, have high common equity, and are highly exposed to a deteriorating housing market, leaving them vulnerable to a deep recession. If they go under and need a large cash infusion from the government, it could cost the country 10% more of GDP to service its debt.

Barron's - Is Fannie Mae the Next Government Bailout?
Wall Street Journal - Fannie, Freddie Could Hurt U.S. Credit
Bloomberg - U.S. Rating Threatened More by Agencies Than Bailouts, S&P Says