They’ve Made Hats for McCain and Trump. And Obama and Harris.

One of the last remaining union-run hat factory in the country is in New Jersey. Over more than three decades, its workers have made hats for competing campaigns.

On an afternoon in September, in a 70,000-square-foot factory in New Jersey, the brims of thousands of camouflage hats were being steamed into a gentle bend so they could be boxed up and shipped across the country.

A large digital display that usually keeps track of how many hats are produced throughout the day had been turned off, because these hats — official merchandise for the Harris-Walz campaign — had been moving down the assembly lines too quickly to count.

“I’ve never seen a surge like this,” said Mitch Cahn, the president of the company, as he walked through the labyrinthine space, where samples and patterns for different bags and caps hang from the ceiling.

Mr. Cahn had been surprised by the popularity of the hats: After Vice President Kamala Harris announced Gov. Tim Walz of Minnesota as her running mate in August, the campaign sold over 50,000 in just a few days. But he hadn’t been caught totally flat-footed.

For more than three decades, Unionwear has been responsible for the merchandise of major presidential campaigns on both sides of the aisle — including for both the McCain and Obama campaigns in 2008 and for Donald J. Trump’s 2016 campaign, when the former president introduced the original MAGA hat. Mr. Cahn still fulfills orders for the unmistakable red hat from third-party groups. (Now considered rare, one circa 2015 Unionwear-produced MAGA hat is listed for $2,000 on eBay.)

Unionwear started making political hats during the 1996 Clinton campaign, but Mr. Cahn said it was the 2000 Gore campaign that started to put merch — and, by extension, where it was made — center stage.