WASHINGTON — Four Democratic senators urged the mortgage companies Fannie Mae and Freddie Mac on yesterday to temporarily freeze foreclosures on loans they hold.

The troubled companies, seized by the government Sunday, should help struggling borrowers swap their mortgages for more affordable loans and stay in their homes, the lawmakers wrote the new chief executives and federal regulator now running Fannie Mae and Freddie Mac.

It was the latest sign of mounting congressional pressure on the director of the Federal Housing Finance Agency, James Lockhart, to ensure that the companies use their clout in the mortgage market to help homeowners caught in the housing crisis.

Senators Schumer, Brown, of Ohio, Casey, of Pennsylvania, and Menendez, of New Jersey — wrote that the companies should “take whatever actions are necessary” so more families “do not have to suffer the economic and personal disaster of foreclosure.”

The firms hold or guarantee some $5 trillion in outstanding mortgages, more than half the nation’s total. The foreclosure freeze, which the lawmakers said should last at least 90 days, would not apply to all those loans.

The Bush administration seized control of the companies in a bid to help reverse a prolonged housing and credit crisis. Fannie Mae and Freddie Mac now are under a conservatorship that ultimately could cost taxpayers billions.

The administration ousted the chief executive of Fannie Mac, Daniel Mudd, and Freddie Mac chief executive Richard Syron and replaced them with a former vice chairman of Merrill Lynch, Herb Allison, and a former vice chairman of U.S. Bancorp, David Moffett, respectively.

Prominent Democrats including the chairman of the House Financial Services Committee, Massachusetts Rep. Barney Frank, have made it clear they expect Fannie Mae and Freddie Mac to do more to help homeowners now.

They have called for Mr. Lockhart to follow the example of the Federal Deposit Insurance Corp. head, Sheila Bair, who has prodded banks to develop comprehensive plans for modifying loans that homeowners can no longer afford. The FDIC temporarily froze foreclosures after it took over the collapsed Pasadena, Calif.-based bank IndyMac. The agency later engineered a plan to allow most IndyMac borrowers who were seriously delinquent or in default on their mortgages to switch into loans capped at an interest rate around 6.5%.

Mr. Frank said this week he was “confident” the administration would push Fannie Mae and Freddie Mac to take similar action. “We will be encouraging Lockhart to do that,” he said.

A spokeswoman for the housing agency said it received the letter and would respond soon.

Republicans and Democrats also are pushing Mr. Lockhart to slash the $24 million in compensation that Messrs. Mudd and Syron stand to gain as they leave their posts.

Mr. Frank’s committee plans a hearing next week examining how the foreclosure prevention law enacted in July is working. Mr. Frank, the chairman of the Senate Banking, Housing and Urban Affairs Committee, Senator Dodd of Connecticut, and Mr. Schumer, who heads the Joint Economic Committee, all plan hearings over the next two weeks on the government’s takeover of Fannie Mae and Freddie Mac.