First and foremost, understand that Fanie Mae and Freddie Mac are responsible in some manner or another for about $5 trillion in debt, or a bit more than 1/3 of the gross domestic product of the USA, and they account for about 55% of all mortgages in the US.

Right now, we appear to be in the midst of a market panic, with their shares, and hence their ability to raise capital to cover bad debt, dropping.

As a result, the Treasury Department is drawing up plans for a government takeover if they prove insolvent, though they are denying any plans for an iminent takeover, though the lesson of Bear Stearns is that going concerns to flat broke can take less than 24 hours.