Washington ? A federal rescue of Fannie Mae and Freddie Mac could cost taxpayers $25 billion, congressional budget experts said, as the House scheduled a vote today on legislation that would tap the mortgage giants’ profits to cover any losses from saving 400,000 homeowners from foreclosure.

A costly rescue for Fannie and Freddie is just a worry, not a certainty at this point. Peter R. Orszag, director of the Congressional Budget Office, predicted in a letter to lawmakers Tuesday that there’s a better than even chance the government will not have to step in to prop up the companies by lending them money or buying stock.

The House was expected to vote today on a housing measure that would give the Treasury Department authority to throw the companies a temporary lifeline.

It’s part of a plan to let hundreds of thousands of strapped homeowners refinance through the Federal Housing Administration into more affordable, government-backed loans at fixed rates rather than losing their homes. The bill would send $3.9 billion to neighborhoods hit hardest by the housing crisis – something that has prompted the White House to threaten a veto.

Taking advantage of the momentum behind the election-year housing package at a time when economic woes top voters’ concerns, Democratic leaders planned to include a separate measure to increase the statutory limit on the national debt by $800 billion, to $10.6 trillion.

The housing bill also would create a new regulator and tighter controls on Fannie Mae and Freddie Mac, the government-sponsored companies that own or guarantee $5 trillion in U.S. mortgages – almost half the nation’s total.

And it would create a new affordable housing fund, which would be drawn from the firms’ profits and cover any losses from the foreclosure rescue plan.