WASHINGTON  Prodded in part by some of the nation’s biggest banks, the Bush administration and Congress are considering costly new proposals for the government to rescue hundreds of thousands of homeowners whose mortgages are higher than the value of their houses.

Not since the Depression has a larger share of Americans owed more on their homes than they are worth. With the collapse of the housing boom, nearly 8.8 million homeowners, or 10.3 percent of the total, are underwater. That is more than double the percentage just a year ago, according to a new estimate of the damage by Moody’s Economy.com.

Administration officials say they still oppose any taxpayer bailout for either people who borrowed more than they could afford or banks that made foolish loans during the height of the speculative bubble in housing.

But with the current efforts to arrest the housing collapse so far bearing little fruit, Washington is being forced to explore new ideas, among them the idea of a federal mortgage guarantee for troubled borrowers.