The U.S. Department of Agriculture is considering buying 400,000 tons of sugar—enough for 142 billion Hershey's Kisses—to stave off a wave of defaults by sugar processors that borrowed $862 million under a government price-support program.

The action aims to prop up tumbling U.S. sugar prices, which have fallen 18% since the USDA made the nine-month operations-financing loans beginning in October. The purchases could leave the price-support program with an $80 million loss, its biggest in 13 years, said Barbara Fecso, an economist...