Shares of mortgage finance companies Fannie Mae and Freddie Mac continued their plunge Wednesday as investors are increasingly convinced that the stocks will drop to zero if the government bails out the troubled companies.

The two government-sponsored companies, the largest source of funding for home mortgages in the U.S., have struggled with soaring losses from mortgage defaults. Washington-based Fannie Mae and McLean, Va.-based Freddie Mac, have lost a combined $3.1 billion between April and June, and investors fear the losses will continue to grow.

Shares in the mortgage finance giants tumbled Wednesday to the lowest levels in nearly two decades. Fannie Mae stock fell $1.08, or 18 percent, to $4.93 in morning trading after earlier hitting a low of $4.80. Shares of Freddie Mac fell 77 cents, or 18.5 percent, to $3.40 after earlier hitting a low of $2.84.

Fannie’s stock is down 87 percent so far this year, while Freddie has lost 90 percent of its value.

“They don’t’ have insight on how bad losses are going to get,” Friedman, Billings, Ramsey & Co. analyst Paul Miller said in an interview Tuesday.

The Bush administration on July 13 unveiled a plan to provide unlimited government loans to the two mortgage giants and to purchase stock in the two companies if needed for a period covering the next 18 months. Congress ultimately adopted those proposals as part of a broader bill that also seeks to help keep 400,000 households from losing their homes to foreclosure.

Critics charged that the open-ended nature of the support for Fannie and Freddie would expose taxpayers to billions of dollars of potential losses.

Treasury Secretary Henry Paulson has insisted that the package needed to be structured in this way to boost financial markets’ confidence as the companies deal with mounting losses from mortgages that have gone bad.

But investors have grown worried in recent days, following a Barron’s magazine article citing an anonymous Bush administration source, reported that the government is pressing the companies to raise more money to guard against losses but doesn’t expect them to succeed.

The Barron’s report said the government is likely to buy preferred stock in the companies, wiping out common shareholders. Paulson has declined to comment on whether a rescue is imminent.