Fannie Mae and Freddie Mac may need as much as $215 billion in additional capital from the Treasury through 2013 to offset losses and maintain a positive net worth, their federal regulator said on Thursday.

Fannie Mae and Freddie Mac, whose programs fund the lion’s share of all new home loans, are at the center of debate as Congress sets to overhaul a U.S. mortgage finance system that contributed to the worst housing crisis since the 1930s.

The cumulative capital needs of the two housing finance giants, which were seized by the government in late 2008, will likely fall between $221 billion and $363 billion through 2013, the Federal Housing Finance Agency estimated.

The projected amounts vary depending on changes in home prices, which in recent years have been the major driver of credit losses for the companies, FHFA said, adding that its exercise is meant to give policymakers “useful snapshots” of the potential need for future taxpayer support.

The FHFA’s lower projection assumes home prices bottomed in the first quarter of 2009, and will rise by 5%  through 2013. The “current baseline” scenario of Moody’s Investors Service depicts more, but smaller house price declines, while a worse outcome reflects a deeper recession because of restricted access to credit and high unemployment, FHFA said.

The companies have drawn $148 billion in the form of preferred stock purchases by the Treasury through the second quarter of 2010.

Under the existing system, Fannie and Freddie shareholders were rewarded during boom times as the companies grew under implicit U.S. support.

Dividend payments on the preferred stock are making up larger portions of the capital needs as time passes, the FHFA said. Of the $73 billion to $215 billion in additional capital that may be needed, $67 billion to $91 billion represent dividend payments to the Treasury, it said.