Since the onset of the credit crisis, the twin pillars of the US housing finance market have been two venerable government-sponsored enterprises: Fannie Mae and Freddie Mac. Now experts are wondering whether a continued decline in house prices could cause these pillars to crack.

If they did, US taxpayers could be forced to foot the bill to shore them up. Fannie and Freddie are private companies but operate under a federal charter. They support the housing market by buying and packaging mortgages into securities stamped with their guarantee, as well as holding large portfolios of these securities themselves.