The trillion-dollar mortgage time bomb

NEW YORK (CNNMoney.com) -- Among the nightmares lurking around the corner for the already battered housing and credit markets would be a meltdown at mortgage financing giants Fannie Mae and Freddie Mac.

Although few are predicting an imminent need for a bailout just yet, credit rating agency Standard & Poor's recently placed an estimated price tag on this worst case scenario -- $420 billion to $1.1 trillion of taxpayer's money.

This dwarfs how much it cost to help banks during the savings and loan crisis of the late 1980's and early 1990's. That cost taxpayers about $250 billion in today's dollars.

S&P added that saving Fannie (FNM) and Freddie (FRE, Fortune 500) might cost so much that the federal government's AAA credit rating, the top possible rating, might even be at risk. If that was lost, then all federal government borrowing would become more expensive.

At the end of January, 82% of all mortgages in the U.S. were backed by one of the firms, up from only 46% in the second quarter of 2007.

"I would say there's at least a 50-50 chance of some sort of bailout. I'm not saying it will necessarily cost $1 trillion, but they'll need some kind of help, and it very well could happen this year," said Dean Baker, co-director of the Center for Economic and Policy Research.