WASHINGTON — Four Democratic senators urged mortgage companies Fannie Mae and Freddie Mac 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 another sign of 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 — wrote that the companies should "take whatever actions are necessary" so more homeowners "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 loans.

A spokeswoman for the housing agency, Stefanie Mullin, said in an e-mail that it had received the letter. "We've been reviewing these issues," she said.

The Bush administration seized control of the mortgage companies in a bid to help reverse a prolonged housing slump and credit squeeze.

Fannie Mae and Freddie Mac now are under a conservatorship that ultimately could cost taxpayers billions.

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