John Marshall:

Fannie and Freddie guarantee loans. What Democrats in Congress did was move Fannie and Freddie aggressively into the subprime market, guaranteeing subprime loans. So, Fannie and Freddie's guarantees of subprimes got them into trouble but, in addition, led others into trouble because Fannie and Freddie's good name was behind the loans. These "others" actually made loans rather than guaranteeing them.

You're right, Fannie and Freddie are partly to blame, although I don't know how aggressively they were actually moved into the subprime market, but private lenders who chose to make what turned out to be very bad loans should also be held responsible for their own actions. If they had really considered where they were putting their money instead of saying "Fannie and Freddie are involved, it must be okay!" the mess might not be a big as it has gotten.