The US Supreme Court is to hear challenges on Friday to President Joe Biden's bid to impose Covid vaccination mandates on millions of American workers.
After months of public appeals to hesitant or reluctant Americans to get their shots, Biden turned up the pressure in September.
"We've been patient, but our patience is wearing thin," he said.
The Democratic president made Covid vaccinations compulsory at businesses that employ 100 workers or more, as well as for health care workers at facilities receiving federal funding.
Unvaccinated employees would have to present weekly negative tests and wear face masks at work.
The Occupational Safety and Health Administration (OSHA) has given businesses until February 9 to be in compliance with the rules or face the possibility of fines.
The vaccine mandates came under immediate attack from some Republican lawmakers and business owners as an infringement on individual rights and an abuse of government power.
A flurry of lawsuits ensued, and the conservative-majority Supreme Court is holding a special hearing to decide whether the mandates can be implemented while the legal challenges continue.
A decision is expected within a few weeks.
Businesses with 100 employees or more represent about two-thirds of the private sector workforce in the United States, or some 80 million people.
The health care worker mandate would apply to roughly 10 million people.
In a brief submitted to the court on behalf of the Biden administration, Solicitor General Elizabeth Prelogar said the measures were necessary "confronted with the deadliest pandemic in the nation's history."
"Workers are becoming seriously ill and dying because they are exposed to the virus that causes Covid-19," Prelogar said.
She said the mandates will save more than 6,500 worker lives and prevent more than 250,000 hospitalizations over the course of six months.
A group of 26 business associations opposed to the mandates, however, said they will "inflict irreparable harm upon hundreds of thousands of businesses."
Companies will be forced to pass the costs of testing on to consumers, resulting in "yet higher prices at a time of record inflation," they argued.
Or businesses will put the costs of testing on unvaccinated employees, they said, "who will quit en masse rather than suffer additional testing costs each week.
"The resulting labor upheaval will devastate already fragile supply chains and labor markets."
Republican-ruled states led by Missouri said forcing health care workers to be vaccinated "threatens to create a crisis in health care facilities in rural America."
"The mandate would force millions of workers to choose between losing their jobs or complying with an unlawful federal mandate," they argued.
The government countered that the number of people who have left their jobs when faced with a vaccination mandate has actually been very low.
At a hospital system in Houston, Texas, for example, just 153 of some 26,000 employees left rather than comply with a vaccination mandate.
Several large US corporations, including meat giant Tyson Foods and United Airlines, imposed vaccination mandates in September without suffering major disruptions.
The Supreme Court has six conservative justices and three liberal justices, and all of them have been vaccinated and received booster shots, according to the court.
The nation's highest court has previously upheld vaccination mandates imposed on college students and health care workers by local authorities.
But the court has previously curtailed federal actions linked to the pandemic, notably by throwing out a moratorium prohibiting evictions.
If the court blocks the vaccination mandates, it would be a major blow to Biden, who has made bringing the pandemic under control one of his priorities but is battling a surge in cases from the Omicron variant.
Vaccination has become a politically polarizing issue in the United States, where 62 percent of the population are vaccinated.
There have been more than 58 million cases of coronavirus in the United States and more than 830,000 deaths.