This opinion piece was first published by The Wall Street Journal and appeared in the September 29, 2021, print edition.

President Biden told unvaccinated Americans this month: “We’ve been patient, but our patience is wearing thin. . . So, please, do the right thing.” He backed up this request with a series of new regulatory mandates, including one from the Occupational Safety and Health Administration, which directs businesses with 100 or more employees to make vaccination a condition of employment.

The Covid vaccine has been widely hailed as a modern scientific miracle. Yet as a means to increase nationwide vaccination rates, the OSHA mandate far exceeds the authority Congress granted the agency, and if the president can order private companies to dictate such terms of employment, his power to coerce citizens in the name of public health might as well be unlimited. This would both be profoundly unconstitutional and fundamentally transform the relationship between the government and the people.

The Occupational Safety and Health Act of 1970 authorizes OSHA to enact rules that are “reasonably necessary or appropriate to provide safe or healthful employment and places of employment.” But the Biden mandate is unreasonably and unnecessarily broad. As announced, it applies to all employees, even those who work at home, as millions have done during the pandemic. It’s simultaneously too narrow, failing to require vaccination for contractors, customers and other nonemployees who may be present at the work site.

It’s overbroad in another way: Previous Covid infection doesn’t excuse employees from the vaccine requirement. Natural immunity tends to be more robust and longer-lasting than vaccinated immunity, according to Marty Makary of the Johns Hopkins University School of Medicine. Worse, Dr. Makary says, there is evidence that people who already have natural immunity are at heightened risk of vaccine side effects caused by an augmented inflammatory response. For these reasons, lawsuits have already been filed challenging employer vaccine mandates as applied to employees with natural immunity.

Another concern is that the administration’s interpretation of the OSHA statutory language presents a “delegation” problem. If Congress delegates discretion to an agency without a proper limiting principle, it violates the separation of powers. To avoid this constitutional problem, the courts will have to give the statute a more restrictive reading. Coming up with a meaningful judicially enforceable principle would not be easy.

Additional problems arise from the administration’s urgency. In imposing the vaccination requirement immediately, OSHA will bypass the ordinary notice-and-comment rule-making process and issue what’s known as an Emergency Temporary Standard. OSHA has used that legal authority only 10 times in 50 years. Courts have decided challenges to six of those standards, nixing five and upholding only one.

The OSH Act imposes stringent limits on emergency standards precisely so OSHA can’t easily circumvent the ordinary rule-making process. The government has to prove that “employees are exposed to grave danger from exposure to substances or agents determined to be toxic or physically harmful or from new hazards” and that using the emergency process is “necessary to protect employees from such danger.” Courts subject emergency standards to a what appellate courts call a “hard look” review, a more stringent standard than for ordinary economic regulations.

The White House justifies the mandate as a proportional response to the spread of Covid’s Delta variant, which is straining hospital capacity in some states. But the mandate is nationwide and indefinite, not tied to Covid rates. The administration’s vaccine rhetoric is therefore another reason to regard the standard as legally suspect. In addition to Mr. Biden’s remark about his patience wearing thin, White House chief of staff Ron Klain retweeted a journalist’s comment that “OSHA doing this vaxx mandate as an emergency safety rule is the ultimate work around for the Federal govt to require vaccinations.”

All this suggests that the administration’s statutory reliance on workplace safety is pretextual. OSHA was established to ensure workplace safety, not to act as a “work around” for achieving other political or policy objectives. In Department of Commerce v. New York (2019), the Supreme Court struck down an otherwise defensible census regulation because the Trump administration’s grounds for instituting it were pretextual.

Beyond these statutory issues lie constitutional concerns. Many commentators are under the impression that Jacobson v. Massachusetts (1905), in which the Supreme Court upheld a vaccine mandate, settles all such questions. But that case involved a state law and a local regulation, not any federal action—a crucial distinction. The states have plenary police power to regulate health and safety. Congress has only those limited powers enumerated in the Constitution. That wouldn’t include the authority to impose a $155 fine (today’s equivalent of the $5 at stake in Jacobson) on an individual who declines to be vaccinated, much less to prevent him from earning a livelihood.

Defenders of the Biden mandate surely will justify it as a delegation pursuant to Congress’s power to regulate interstate commerce. But the actual target of the rule is individual medical choices, not commercial ones. If a personal decision not to buy medical insurance can’t be characterized as “commerce”—as the Supreme Court held in NFIB v. Sebelius (2012), the ObamaCare case—how can the decision not to be vaccinated?

Further, if public-health benefits are sufficient to justify an OSHA vaccine mandate, what principle would limit the agency’s authority? Could it ban employees from smoking or consuming foods containing trans fats while working at home? The public-health profession has already characterized everything from gun ownership to social-media use as posing a serious public-health issue. Could OSHA legitimately police these, too, even away from the workplace?

Higher vaccination rates would be a public good. But our nation’s Founders understood that much mischief can be done under the theory of being “for your own good” and provided limits to government authorities accordingly. Even during a pandemic, the Biden administration would do well to respect those limits.

Mr. Rivkin practices appellate and constitutional law in Washington. He served at the Justice Department and the White House Counsel’s Office in the Reagan and George H.W. Bush administrations. Mr. Alt is president and CEO of the Buckeye Institute, a think tank engaged in public-interest litigation and policy.