From CNN/Money:

The trillion-dollar mortgage time bomb

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 and Freddie 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.

Wagner pointed out that 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.

And Fannie and Freddie’s role in the mortgage and real estate markets is likely to grow, as Congress recently allowed them to back larger mortgages, up to $729,750, up from the previous limit of $417,000.

“I don’t think the message is a bailout is necessary or imminent,” Wagner said. “But they’re facing this increased role at a time that their own credit performance is suffering from the rifts in the housing and mortgage markets. They’re both projecting much higher losses than we’ve seen in some time.”

“The real fundamental problem is real estate prices have been falling and they might fall substantially more,” said Robert Shiller, a Yale University economist who argued for years that a bubble was forming in real estate prices. “OFHEO and Fannie and Freddie never considered the possibility of a massive real estate correction.”

“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.