From the Economist (emphasis mine):
After a headlong plunge in the two firms’ share prices (see chart 1), Hank Paulson, the treasury secretary, felt obliged to make an emergency announcement on July 13th. He will seek Congress’s approval for extending the Treasury’s credit lines to the pair and even buying their shares if necessary. Separately, the Federal Reserve said Fannie and Freddie could get financing at its discount window, a privilege previously available only to banks.
The absurdity of this situation was highlighted by the way the discount window works. The Fed does not just accept any old assets as collateral; it wants assets that are “safe”. As well as Treasury bonds, it is willing to accept paper issued by “government-sponsored enterprises” (GSEs). But the two most prominent GSEs are Fannie Mae and Freddie Mac. In theory, therefore, the two companies could issue their own debt and exchange it for loans from the government—the equivalent of having access to the printing press.
That would certainly solve the problem of fewer open-market buyers for the GSEs’ paper.