As home prices decline and Washington struggles to end the economic malaise, Wall Street is starting to send a sobering message: The worst is yet to come.

One of the strongest warning signs came Monday, when shares of the nation’s most important mortgage companies, Fannie Mae and Freddie Mac, plummeted. After falling almost continuously over the past month, in just one day Freddie Mac tumbled another 18 percent, and Fannie Mae lost 16 percent amid concerns that the companies would need to raise billions of dollars in fresh capital.

Fannie Mae and Freddie Mac are the nation’s largest buyers of home mortgages, and traditionally the government’s backstop for the housing economy. But with Monday’s plunge, each of these giants has now lost more than 60 percent of its market value this year. The declines, along with a falling stock market and growing unease about the possibility of more red ink at big banks, reflect a growing conviction consensus among investors that the current housing slump will last longer, and prove more severe, than initially feared.

As a result, investors are signaling that they are far from convinced that any enterprise  even ones with the strongest backing  can successfully navigate these choppy waters, and that those who do survive will pay dearly.