The U.S. Capitol seen from an empty meeting room in the Hart Senate Office Building during the federal government shutdown.

The economic effect of past government shutdowns has been straightforward. The economy loses some activity for a few weeks, then gains it back after the government reopens. The net cost is basically zero.

This time, the math may not be so benign.

As Washington’s stalemate continues into its fourth week with no end in sight, it’s looking like this could become one of America’s longest funding lapses. During the previous record-holder, a 34-day closure in 2018, Congress passed enough appropriations bills to keep more of the government funded. This time, none have been passed.

And the White House is attempting to lay off thousands of people and threatening to withhold back pay for furloughed workers, despite a 2019 law requiring that they be paid. “That would obviously make it a larger macroeconomic impact,” said Michael Zdinak, a director on the United States economics team at S&P Global Market Intelligence.

Then there are the services those workers aren’t providing, including national park tours and new drug reviews, that support commerce. For many businesses, the timing couldn’t be worse, with the holiday season approaching and economic uncertainty already high.

“If you’re worried about the potential for those indirect impacts, those only increase the longer the shutdown goes on,” Mr.