The Treasury won't necessarily make a large injection of capital immediately into the ailing companies, which provide the bulk of funding for U.S. home mortgages. But people familiar with the plan said the Treasury will stand ready to provide capital as needed, depending how quickly losses deplete the companies' meager capital holdings.

The Federal Housing Finance Agency, Fannie and Freddie's regulator, is to use its legal powers to put the companies under conservatorship. Those powers allow the FHFA to run the companies indefinitely, under certain conditions, such as when the regulator finds that they are likely to be unable to meet their financial obligations. Fannie and Freddie have run up combined losses totaling about $14 billion over the past four quarters and face heavy additional losses amid the worst surge in U.S. home-mortgage foreclosures since the 1930s.