“We’re seeing core consumers under a lot of pressure,” Duke said at an event in New York. “There’s no doubt that rising fuel prices are having an impact.”

Wal-Mart shoppers, many of whom live paycheck to paycheck, typically shop in bulk at the beginning of the month when their paychecks come in.

Lately, they’re “running out of money” at a faster clip, he said.

“Purchases are really dropping off by the end of the month even more than last year,” Duke said. “This end-of-month [purchases] cycle is growing to be a concern.