The Federal Reserve is only allowed to purchase debt that is backed by the full faith and credit of the US government.
The face page of (housing) Fannie debt and Freddie debt explicity states that it is not guaranteed by the US government.
In Congressional hearings, Ron Paul had Ben Bernanke admit the Fed has bought Fannie and Feddie debt. The Fed literally created money out of nothing to buy debt the Fed is not allowed to buy.
The Fed broke it's charter. Audit The Fed! End The Fed!
