As home prices continue their free fall and banks shy away from lending, Washington officials have increasingly relied on two giant mortgage companies  Fannie Mae and Freddie Mac  to keep the housing market afloat.

But with mortgage defaults and foreclosures rising, Bush administration officials, regulators and lawmakers are nervously asking whether these two companies, would-be saviors of the housing market, will soon need saving themselves.

The companies, which say fears that they might falter are baseless, have recently received broad new powers and billions of dollars of investing authority from the federal government. And as Wall Street all but abandons the mortgage business, Fannie Mae and Freddie Mac now overwhelmingly dominate it, handling more than 80 percent of all mortgages bought by investors in the first quarter of this year. That is more than double their market share in 2006.

But some financial experts worry that the companies are dangerously close to the edge, especially if home prices go through another steep decline. Their combined cushion of $83 billion  the capital that their regulator requires them to hold  underpins a colossal $5 trillion in debt and other financial commitments.