They are gigantic mortgage lenders that control trillions of dollars and back about 80 percent of our nation's mortgages.

On Monday morning, the federal government is stepping in to make sure Freddie Mac and Fannie Mae do not fail.

Shares of the government's two giant mortgage backers have plunged as losses from mortgage holdings surged. But a new plan from the treasury department could help.

Under it, the companies can now borrow directly from the Federal Reserve at the same rate as commercial banks.

The treasury is also planning to expand its current line of credit to both companies if they need it. These moves still need congressional approval. The two companies either hold or back $5.3 trillion of mortgage debt.

What does this all mean? Howard Dicus explains:

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