Carhartt, the work wear and outdoor gear maker, is facing calls for a boycott among conservatives opposed to its vaccine mandate for staff.
The outcry comes amid fierce debate in the US over Covid-19 jabs.
This month, the US Supreme Court said the government could not force large employers to make Covid-19 vaccination or weekly testing mandatory.
Companies including the coffee chain Starbucks have since reversed their plans for such rules.
But in a recent memo to staff, Carhartt leaders said the Supreme Court decision had not changed company requirements, which went into effect for much of their staff in early January.
"We put workplace safety at the very top of our priority list and the Supreme Court's recent ruling doesn't impact that core value," wrote Carhartt's chief executive Mark Valade, a descendant of the company's founder.
"While we appreciate that there may be differing views, workplace safety is an area where we and the union that represents our associates cannot compromise. An unvaccinated workforce is both a people and business risk that our company is unwilling to take. "
After copies of the message were circulated on social media, prominent conservatives and Republican figures weighed in.
"Well Carhartt has labelled the unvaccinated dangerous and implemented a vaccine mandate," conservative attorney Molly McCann wrote on Twitter.
"Pretty rich from a company sustained by the ranchers, farmers, labourers, etc. who make this country great and celebrate her values of freedom and liberty."
"Boycott Carhartt until they break," she said.
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Founded in 1889, Carhartt employs more than 5,500 people globally. It is privately-owned and based in Michigan, a state in the middle of the US that is fiercely divided between Republicans and Democrats.
Self-promoted as a brand for "hard-working" people, Carhartt made its name supplying labourers and farm workers. But it has also been embraced by fashionistas in Europe, where it has a longstanding "Work in Progress" design line.
In recent years, which the firm has described as a "growth period", Carhartt caps and coats have turned up everywhere from playgrounds to the Golden Globes. A 2017 article in men's magazine Esquire described it as one of the rare brands able to transcend America's political divide - at least so far.
"Regardless of the boycott, when you think about core American fashion brands, people always mention Ralph Lauren and Levi, but Carhartt is also on that list," said Jeff Carvalho, co-founder of men's fashion site Highsnobiety. "They're just quite important."
Mr Carvalho, a collector of vintage Carhartt gear, said conservatives represent an important consumer base for the brand, but he believes it will survive the current firestorm, noting that buyers have few alternatives. He asked: "Who are they going to turn to otherwise?"
"It is the uniform, or tuxedo of American industry."
Carhartt, which has factories in Kentucky and Tennessee as well as Mexico, said the "vast majority" of its staff are fully vaccinated or in the process of getting their jabs. It also granted "a number" of religious and medical exemption requests.
Getty ImagesModel Kaia Gerber steps out in a Carhartt jacketA spokeswoman for the company said: "Carhartt fully understands and respects the varying opinions on this topic, and we are aware some of our associates do not support this policy.
"However, we stand behind our decision because we believe vaccines are necessary to protect our workforce."
Just over a third of US workers are now covered by vaccine mandates, according to a Gallup poll in December.
While the Supreme Court struck down the national requirement, courts have been more open to states and companies setting their own policies.
Investment bank Citigroup and United Airlines are among the firms that have moved forward with vaccine mandates. United Airlines's boss Scott Kirby recently told staff that the policy had saved lives.
Other firms have wavered in the face of opposition.
About 55% of US workers support vaccination requirements at work, a Gallup poll found in December. But more than a third were strongly opposed. Divide over the issue is linked to politics, with Republicans leading the opposition.
Business groups say companies are worried about compliance costs and staff recruitment. About 63% of Americans are fully vaccinated but that number varies significantly depending on location.
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