Washington, D.C. ? Four Democratic senators urged the mortgage companies Fannie Mae and Freddie Mac on Thursday 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 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 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.