The number of Americans who have seen their full-time jobs chopped to part time because of weak business has swelled to more than 3.7 million  the largest figure since the government began tracking such data more than half a century ago.

The loss of pay has become a primary source of pain for millions of American families, reinforcing the downturn gripping the economy. Paychecks are shrinking just as home prices plunge and gas prices soar, furthering the austerity across the nation.

“I either stop eating, or stop using anything I can,” said Marvin L. Zinn, a clerk at a Walgreens drugstore in St. Joseph, Mich., who has seen his take-home pay drop to about $550 every two weeks from about $650, as his weekly hours have dropped to 37.5 from 44 in recent months.

Mr. Zinn has run up nearly $2,000 in credit card debt to buy food. He has put off dental work. He no longer attends church, he said, “because I can’t afford to drive.”