WASHINGTON — President Joe Biden’s far-reaching assertion of executive authority to require COVID-19 vaccines for 100 million U.S. workers relies on a set of complicated legal tools that will test the power — and the limits — of the federal government to compel personal health care decisions.

To more aggressively confront the coronavirus pandemic, Biden is pulling several levers of presidential power: He is using an emergency provision in the Occupational Safety and Health Act of 1970; he is threatening to withhold federal funding from hospitals and other health care organizations; and he is embracing his authority as chief executive of the sprawling federal workforce and its contractors.

Together, the president’s actions are an assertive use of his jurisdiction over U.S. life as the occupant of the Oval Office. Until Thursday, under Biden’s leadership, the White House had been far more cautious about mandating vaccines than his counterparts around the world, especially in Europe.

On Friday, facing accusations from Republicans of an abuse of power and threats of lawsuits, Biden had a simple retort.

“Have at it,” he said.

The right of government to impose vaccines has been established since at least 1904, when the Supreme Court issued a 7-2 ruling that Cambridge, Massachusetts, could require all adults to be vaccinated against smallpox. But more recent cases — including the first Supreme Court ruling on the Affordable Care Act — call into question whether Biden or any president could simply order all Americans to get shots.

That is not what Biden is doing. By requiring that companies maintain safe workplaces through vaccination, legal experts said Friday that the president was relying on the federal government’s well-established constitutional power to regulate commerce and the 51-year-old law establishing the Occupational Safety and Health Administration.

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Once OSHA drafts an “emergency temporary standard” and a White House regulatory office reviews it, officials said the agency would begin enforcing the rules: collecting reports of violations and sending out inspectors who will be empowered to impose $13,650 fines for violations and up to $136,500 for those that are willful or repeated.

“The constitutionality of this regulatory effort is completely clear,” said Donald B. Verrilli Jr., who served as solicitor general under President Barack Obama. “In a situation like this, one where we’re in the middle of a public health emergency, courts recognize that they lack the institutional competency to make judgments about what’s in the best interest of public health and safety.”

Biden’s adversaries are already accusing him of an abuse of power, saying he has gone too far in confronting a virus that has claimed the lives of more than 600,000 Americans. Sen. Ted Cruz, R-Texas, called the president’s actions “utterly lawless.” Gov. Brian Kemp, a Georgia Republican, said the move was “blatantly unlawful, and Georgia will not stand for it.”

In a fundraising email sent Friday, Gov. Ron DeSantis of Florida, a Republican who has issued anti-mask orders, wrote, “Joe Biden has declared war on constitutional government, the rule of law, and the jobs and livelihoods of millions of Americans.”

But top aides to the president do not appear to be shaken by what they say was an expected response from those quarters. White House officials believe he has clear authority to compel federal workers to be vaccinated as a condition of their employment by the government. And they say requiring hospitals and other health care organization to vaccinate its workers — a mandate that covers as many as 17 million people — is a reasonable condition in exchange for taking federal health care reimbursements.

The most novel part of the president’s announcements Thursday relate to his use of the emergency authority provided by Congress under the Occupational Safety and Health Act of 1970 as a way to require most U.S. workers to submit to vaccination against the virus.

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More on the COVID-19 pandemic

White House officials said OSHA was likely to take at least three or four weeks to write the new standard, partly because it must complete certain time-consuming steps to ensure that the rule passes legal muster. Among them are rigorously demonstrating that workers face a grave danger at work, that the rule is necessary to defuse that danger and that it is feasible for employers to carry out.

OSHA must also sort through a number of practical questions, such as who pays for the testing and what kinds of tests are acceptable.

In the case of the COVID-19 vaccine, the administration will argue that the death and illness caused by the delta variant of the coronavirus poses a “grave danger” to workers across the country, and that the vaccine is an extremely effective way of preventing severe illness, hospitalization and death.

Those arguments are likely to be included as part of a preamble to the regulatory language that officials at OSHA and the Labor Department are drafting, according to a White House official, who spoke on the condition of anonymity to discuss regulations that are still under development.

Former OSHA officials in touch with former colleagues there said the agency only learned about plans for the standard during roughly the past week, so current OSHA officials did not have a chance to prepare extensively before Biden’s announcement.

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“The White House is asking OSHA how fast they can do it, and OSHA said, ‘Who the hell knows?’ ” said Jordan Barab, a deputy director of the agency under Obama. “They only had a week’s notice.”

A White House official said Labor Secretary Martin J. Walsh was central to the development of the policy.

The agency has clear authority to issue an emergency standard. But before issuing an emergency coronavirus standard in June that required employers to provide protective equipment like masks and ensure adequate distancing and ventilation, the agency had not tried to use that authority in decades. Some attempts in the 1970s and early ’80s, such as an emergency standard related to asbestos, were set aside by judges who said the standard did not clear the required legal thresholds.

OSHA has not previously written a standard requiring vaccinations, though a 1991 rule required that certain employers make the hepatitis B vaccine available to workers who could come into contact with blood while doing their jobs, such as dentists and hygienists.

Once the rule is published in the Federal Register, employers are likely to have at least a few weeks to comply before OSHA begins enforcement, though they may have to put forth a policy even sooner. Enforcement could happen in a number of ways: The agency could prioritize a single problematic industry or industries and focus inspections there. It could also conduct inspections in response to news reports of outbreaks or worker complaints.

And because the rule is likely to require employers to keep records of which workers are vaccinated and which have recently been tested, the agency could ask inspectors following up on unrelated concerns to check records for compliance with the vaccination rule.

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But OSHA only has a small number of inspectors relative to the size of the workforce. A recent report by the National Employment Law Project, an advocacy group, found that the agency had fewer than 900 inspectors as of Jan. 1, 2020, down from over 1,000 in 2012, and that it would take more than 150 years to conduct a single inspection of each workplace under its jurisdiction.

While the coronavirus relief plan Biden signed in March provides funding for additional inspectors, few if any are likely to be deployed by the end of this year.

That means enforcement is likely to be strategic, focusing on a tiny number of high-profile cases where large fines can attract attention and send a message to other employers. A workplace that failed to enact the vaccination or testing requirement could in principle pay a fine for each worker affected, though OSHA rarely proposes such aggressive fines.

But even absent aggressive enforcement, employers tend to follow OSHA rules.

Experts said the administration appeared to be on strong legal ground.

“The OSHA Act gives employees a right to a safe and healthy workplace,” said Robert I. Field, a law professor at Drexel University in Philadelphia. “Having a vaccinated workforce is an essential component of having a safe and healthy workplace.”

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Biden’s call to use that authority was a sharp shift in tone and approach. For months, the president tried gentle persuasion. Anything more, the White House worried, would backfire in a polarized country where tens of millions of people viewed the COVID-19 vaccine as a political Rorschach test.

But on Thursday, he declared himself out of patience with the unvaccinated.

“This is not about freedom, or personal choice,” he said. “It’s about protecting yourself and those around you.”

The argument may provoke exactly the kind of blowback Biden’s team worried about.

“Federal government mandates, of dubious legality, will further alienate the skeptical, undermine our institutions, and punish ordinary business owners and their employees,” Sen. Pat Toomey, R-Pa., said Friday.

But in a statement, White House officials said the president was “committed to pulling every lever possible” in the fight against the pandemic.

“This action is both clearly legal and needed to help save lives and stop the spread of COVID-19,” it said.