Updated

Congressional leaders seized on the enormous implications for taxpayers and the economy as the historic takeover of Fannie Mae and Freddie Mac could come as soon as Sunday.

Fannie Mae and Freddie Mac together hold or back half of the nation's mortgage debt, and have played an increasingly important role in the real estate market since the credit crisis started in August 2007. A government bailout could cost taxpayers around $25 billion, according to the Congressional Budget Office.

Treasury Secretary Henry Paulson and two other regulators are working on a plan to put the troubled mortgage finance companies into a conservatorship, and remove Fannie Mae CEO Daniel Mudd and Freddie Mac CEO Richard Syron, according to Rep. Barney Frank, D-Mass., the chairman of the House Financial Services Committee.

The government is expected to control the two companies at least a year as it evaluates and debates whether Fannie and Freddie should remain government-run entities or be restructured in some fashion, Frank said in an interview.

News of the likely government takeover Friday followed a report by the Mortgage Bankers Association that more than 4 million American homeowners with a mortgage, a record 9 percent, were either behind on their payments or in foreclosure at the end of June.

That confirmed what investors saw in Fannie and Freddie's recent financial results: trouble in the mortgage market has shifted to homeowners who had solid credit but took out exotic loans with little or no proof of their income and assets.

Fannie Mae and Freddie Mac lost a combined $3.1 billion between April and June. Half of their credit losses came from these types of risky loans with ballooning monthly payments.

While both companies said they had enough resources to withstand the losses, many investors believe their financial cushions could wither away as defaults and foreclosures mount.

Frank said the companies' financial picture was better than Wall Street investors assumed, but "it just plainly became clear that elements of the market wouldn't' accept that."

The epic decision highlights the size of the threats facing the housing market and the economy. On Friday, Nevada regulators shut down Silver State Bank, the 11th failure this year of a federally insured bank. And earlier this year, the government orchestrated the takeover of investment bank Bear Stearns by JP Morgan Chase.

"Any government action must help to strengthen our economy, which is suffering a crisis brought on by the administration's failure to stop predatory lending," said Sen. Chris Dodd, D-Conn., who chairs the Senate Committee on Banking, Housing, and Urban Affairs. "Any intervention also must minimize the cost to American taxpayers, and should not put other financial institutions at risk."

The crisis surrounding Fannie and Freddie promises to be a major challenge for the next president.

The role the two companies play in the U.S. mortgage market has grown dramatically over the past year as other lenders collapsed under the weight of bad subprime loans. The companies guaranteed about three-quarters of all new mortgages in the second quarter of this year, up from under 40 percent in 2006, according to the trade publication Inside Mortgage Finance.

Federal Reserve Chairman Ben Bernanke, Treasury Secretary Henry Paulson and James Lockhart, the companies' chief regulator, met Friday afternoon with the top executives from the mortgage companies and informed them of the government's plan to put the companies into a conservatorship as early as this weekend.

In July, Congress passed a plan to provide unlimited government loans to Fannie and Freddie and to purchase stock in the companies if needed. Critics say the open-ended nature of the rescue package could expose taxpayers to billions of dollars of potential losses.

Fannie Mae was created by the government in 1938, and was turned into a public company 30 years later. Freddie Mac was established in 1970 to provide competition for Fannie.