NEW YORK—Wall Street tumbled Wednesday as investors grappled with renewed worries about the soundness of the financial sector. The major indexes fell more than 2 percent, including the Dow Jones industrial average, which lost more than 230 points.

While many financial services companies logged steep declines during the session, government-sponsored lenders Freddie Mac and Fannie Mae were among those hardest hit. Investors are worried that the mortgage finance companies will have to sell more shares than anticipated to compensate for losses from the housing slump. Merrill Lynch & Co...