Four Democratic senators urged the mortgage companies Fannie Mae and Freddie Mac on Thursday to freeze foreclosures for 90 days 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 the federal regulator now running Fannie Mae and Freddie Mac.

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

The senators — Sherrod Brown of Ohio, Bob Casey of Pennsylvania, Bob Menendez of New Jersey and Charles Schumer of New York — want the companies to "take whatever actions are necessary" so more families "do not have to suffer the economic and personal disaster of foreclosure."

The companies hold or guarantee some $5 trillion in outstanding mortgages, more than half the nation's total. The foreclosure freeze would not apply to all 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 Fannie Mac chief executive Daniel Mudd and Freddie Mac chief executive Richard Syron and replaced them with Herb Allison, a former vice chairman of Merrill Lynch, and David Moffett, a former vice chairman of U.S. Bancorp, respectively.

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

They have called for Lockhart to follow the example of Sheila Bair, the Federal Deposit Insurance Corp. head, who has prodded banks to develop comprehensive plans for modifying loans that homeowners can no longer afford. After the collapse of Pasadena, Calif.-based IndyMac bank, temporarily froze foreclosures after her agency took over IndyMac. The FDIC 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 percent.

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.

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

Frank's committee plans a hearing next week examining how the foreclosure prevention law enacted in July is working. Frank and Democratic Sen. Chris Dodd of Connecticut, chairman of the Senate Banking, Housing and Urban Affairs Committee, expected to hold hearings soon on the government's takeover of Fannie Mae and Freddie Mac.