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.

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.”

These workers now amount to 3.7 percent of all those employed, up from 3 percent a year ago, and the highest level since 1995.

“This increase is startling,” said Steve Hipple, an economist at the Labor Department.