WASHINGTON — Shares of Freddie Mac plummeted today as Wall Street and Washington became more convinced that the government is likely to bail out the nation’s key mortgage financiers.

Freddie Mac shares fell $3.30, or 41.3%, to $4.70 in morning trading, while Fannie Mae fell $4.90, or 37.1%, to $8.30. Both are at 17-year lows.

“I think everybody’s just holding their breath in expectation that something substantive from the government will happen today or over the weekend,” a managing partner of consulting firm Federal Financial Analytics at Washington, Karen Shaw Petrou, said.

The companies’ troubles are more a result of market perceptions than a changed financial picture at the two companies, Ms. Petrou said.

“External reality doesn’t warrant such an action, but external reality seems no longer to matter,” she said.

Under a 1992 law, if either company fell into financial trouble, the government could take over their operations by placing it in a conservatorship.

A Wachovia Corp. economist, Jay Bryson, said the two mortgage giants could face a replay of the near-collapse in March of investment bank Bear Stearns Cos. A lack of market confidence could make it difficult for Fannie and Freddie to raise funding through debt sales, he said.

“It becomes a liquidity issue, rather than a solvency issue,” Mr. Bryson said.

The New York Times reported today the government was considering taking over the operations of one or both of the companies, adding to fears that have mounted this week. Representatives from Fannie and Freddie were not immediately available for comment this morning.

Fannie and Freddie play a crucial role in providing funding for home loans by buying up mortgages and packaging them as investments. If they are unable to operate, the implications could be dire.

“Without them, our economy would collapse,” a Piper Jaffray analyst, Robert P. Napoli, said in a note to clients. Mr. Napoli lowered his target on Freddie to $9 per share from $28, and on Fannie to $15 per share from $30.

Yesterday, the Office of Federal Housing Enterprise Oversight — the companies’ chief regulator — said both Fannie and Freddie remain “adequately capitalized,” after the Treasury secretary, Henry Paulson, and Federal Reserve chairman, Ben Bernanke, sought to calm investors jitters in testimony on Capitol Hill.

Yet reassurances by government officials appear not to be working.

“We doubt anyone will listen as fear is so high,” Mr. Napoli said.

Congress created Fannie in 1938 and Freddie in 1970 to keep money flowing into the home-loan market by buying up mortgages and bundling them into securities for sale to investors worldwide — thereby making home ownership affordable for low- and middle-income Americans.

Today the companies hold or guarantee around $5.3 trillion in home-loan debt, though under a 1992 law they are required to hold only a fraction of what is mandated for commercial banks as a financial cushion against risk.

A Friedman, Billings, Ramsey & Co. analyst, Andrew Parmentier, said the question of capital-raising plans at either company remains a “moving target.”

“In an instance where equity capital is not raised and investors see a meaningful change in debt spreads, it is clear to us that government action would be undertaken to ensure that the institutions would not fail,” Mr. Parmentier said in a note to clients.