NEW YORK—Wall Street retreated Monday after Fannie Mae and Freddie Mac fell to their lowest levels in nearly 20 years on concerns that the government might need to bail out the mortgage financiers. Weakness in the overall financial sector sent the Dow Jones industrial average down more than 175 points.

Investors were again uneasy about the health of financial companies after media reports of further problems in the sector. Barron’s said the U.S. Treasury might have to bail out government-chartered Fannie and Freddie, which, the weekly noted, would likely wipe out shareholders’ equi...