The Biden administration on Thursday set Jan. 4 as the deadline for large companies to mandate coronavirus vaccinations or start weekly testing of their workers, the government’s biggest effort yet to enlist private businesses in combating the virus.

The new rule, applying to companies with 100 or more employees, is expected to cover 84 million workers, roughly 31 million of whom are unvaccinated. It lays out details of a plan President Biden announced in September, invoking emergency powers over workplace safety.

In a separate measure that will affect 17 million more workers, nursing homes and other health care facilities that receive Medicare and Medicaid funds must ensure all employees are vaccinated by Jan. 4, with no option for testing. The president has previously imposed vaccine requirements on federal workers, a group that totals more than four million people, and companies that have federal contracts. (The latter group’s deadline was pushed to Jan. 4, from early December.)

But the mandate on large private businesses is the most far-reaching and potentially controversial measure in the government’s efforts to fight the pandemic. Attorneys general in at least 24 states have threatened to sue. Republican governors and some industry trade groups have opposed the requirement, and the 20 percent of U.S. adults who remain unvaccinated may take issue as well.

And the administration is considering going even further. The Labor Department said it had opened a 30-day comment period on whether it should extend the rules to smaller companies.

“While I would have much preferred that requirements not become necessary, too many people remain unvaccinated for us to get out of this pandemic for good,” Mr. Biden said in a statement.

Still, with the country facing shipping delays and shortages caused by supply chain problems, the January deadline allows retailers and logistics companies, both in need of employees, to get through the holiday shopping season before instituting the requirements.

The rule does, however, instruct employers to require masks for unvaccinated workers by Dec. 5. Companies must provide paid time off for vaccinations and sick leave for side effects as needed. The requirement does not apply to remote and outdoor workers. Some major companies including Tyson Foods and United Airlines have already embraced vaccine mandates. But for many that have been fearful of resistance, the new requirement provides cover to enforce mandates and clarification on a range of questions, including who will pay for testing and whether it applies to employees who work at home.

With many of those questions now answered by the requirements and detailed guidance published by the Occupational Safety and Health Administration, many more businesses are expected to soon announce mandates.

Among those without vaccine requirements are the nation’s largest private employer, Walmart, whose mandates apply mainly for its corporate staff, and JPMorgan Chase, which has over 120,000 employees in offices and bank branches across the United States and is encouraging but not broadly mandating the shots.

In a Mercer poll of 1,088 companies conducted on Oct. 4, roughly 13 percent of respondents said they were requiring all employees to be vaccinated, regardless of work location. Eleven percent said they were requiring shots only of those coming to the office.

According to OSHA’s new requirements, workers are considered fully vaccinated if they have received two doses of the Pfizer-BioNTech or Moderna vaccines, one dose of the Johnson & Johnson vaccine, or another vaccine approved for emergency use by the World Health Organization. Companies can verify a worker’s status by requesting either a vaccination card or proof from a medical provider. Alternatively, employees can provide a signed note pledging that they have been vaccinated.

OSHA will allow exceptions for medical or religious reasons. It estimates that 1 percent of workers who remain hesitant have a medical reason, and 4 percent have a religious objection.

Employers are not required to either pay for or provide tests, though some may still be compelled to do so by other laws or agreements with unions. Forcing unvaccinated employees to pay for tests, the rule notes, “will provide a financial incentive for some employees to be fully vaccinated.”

Companies that fail to comply may be fined. An OSHA penalty is typically $13,653 for every serious violation, but can be up to 10 times that amount if OSHA determines that the violation is willful or repeated.

OSHA approaches enforcement in a number of ways: It can target inspections at high-risk industries, or it can inspect workplaces in response to complaints or news reports about unsafe conditions.

Employers must provide information about the total number of fully vaccinated workers to employees or their unions, which the agency believes will aid compliance. Employees and unions can use this information to pressure companies directly, or as part of the basis for a complaint.

As a practical matter, however, inspectors are likely to play only a small role in enforcement given the agency’s personnel constraints. A 2020 report by the National Employment Law Project, a worker advocacy group, found that the agency had only about 860 inspectors at the beginning of that year, down about 15 percent from 2012.

The relief bill that Mr. Biden signed into law this year provides funding for additional inspectors, but it could be many months before most of them are hired and sent into the field.