Eugene Scalia, a Washington lawyer, served as labor secretary under President Donald Trump and as Labor Department general counsel under President George W. Bush.

The vaccine mandates President Biden announced recently are causing uncertainty for workers and employers — and that’s likely to persist as legal challenges are filed that could result in the mandates being set aside.

For private-sector companies, the president announced two principal actions: First, an emergency temporary standard (ETS) from the Labor Department’s Occupational Safety and Health Administration (OSHA), which reportedly will require employers with 100 employees or more to mandate vaccines for workers, or require them to present a negative coronavirus test weekly. Employers will have to provide paid time off for employees to get vaccinated and to recuperate from the vaccination, if needed.

Still unclear: What documentation of vaccination and testing will OSHA require? Will it order employers to pay for coronavirus tests for workers who choose that route? Will those working from home, or those with “natural” immunity, have to be vaccinated? Will vaccinated workplaces be exempted from mask mandates? And what role will unions play in negotiating implementation? OSHA’s rule is expected within weeks. A swift judicial stay of the rule is possible, followed by months — at least — of litigation.

Second, the president directed a White House task force to develop, by Sept. 24, covid-19 requirements for employers that contract or subcontract with the federal government. What the task force will order is even less clear, though one would expect its requirements to be harmonized with OSHA’s.

Unlike some presidential directives, the forthcoming OSHA rule has a legal toehold: Emergency temporary standards are statutorily authorized when “necessary” to address “grave danger” in the workplace. (The rule may be in place for six months, during which time OSHA must develop and issue a “permanent” rule.) OSHA already issued a covid-related emergency temporary standard in June, imposing special protocols on the health-care sector. I decided against a covid-19 ETS when I was labor secretary, not because I concluded there was no legal authority but because I thought there were superior ways to address the pandemic in the workplace. Biden retained that approach, except for the health-care sector, until the Pfizer-BioNTech vaccine received full FDA approval.

Of course, having a legal toehold doesn’t mean a rule will survive in court. OSHA historically has a poor record of defending emergency temporary standards.

That’s partly due to procedural concerns. Unlike most rules by government agencies, an ETS is issued without prior public review and comment. A public comment period confers more democratic legitimacy on administrative rules. It also improves the rules substantively. Courts appropriately review purportedly “emergency” rules more skeptically.

So, in the case of the vaccine ETS, courts will ask why it was “necessary” to preclude public comment, and in one obvious practical sense, it’s not: To help shield its rule from legal challenge, OSHA can release a proposed rule and let the public comment, even if it doesn’t conduct full-blown notice-and-comment rulemaking. Doing so may not satisfy courts’ procedural concerns but would produce a substantively better and more defensible rule, informed by the insights of those who must live under it.

Expect procedural objections to the OSHA rule to be accompanied by substantive challenges. No one can assess all of a rule’s legal vulnerabilities before release; we have to see what OSHA requires and the evidence it offers in support. OSHA must demonstrate “grave danger” that can be addressed only by an immediate vaccination mandate. And it must explain why this grave danger doesn’t require the same response for workplaces with 99 workers or, if costs are the reason, why those costs are tolerable for companies with 200 workers.

Other potential vulnerabilities abound, including how the president rolled out the rule: He presented it as a nationwide public health initiative in which employers, in effect, would be dragooned via OSHA as a means of boosting overall vaccination rates. OSHA’s authority is for grave workplace dangers, not nationwide public health campaigns. In litigation, OSHA will find itself walking back the president’s explanation.

Meanwhile, the mandate for government contractors faces potentially higher hurdles. The president is using his authority under the Procurement Act to promulgate covid-19 requirements for employers doing business with the government. And it won’t even be the president who sets the requirements but a White House task force. This simply is not how law is made in this country; this particular overreach could bring an overdue judicial rebuke to Republican and Democratic presidents’ use of the procurement power to set employment policy.

I join Biden in urging Americans to protect themselves and others through a vaccination program that is one of President Donald Trump’s most important achievements. OSHA has a valuable role in preventing covid-19 transmission at work, and many employers have reasonably decided to require that their employees be vaccinated. But in upbraiding unvaccinated Americans, and telling them that he will put their employer and OSHA on their back if they don’t get in line, the president gave a shaky launch to an initiative that was already bound for litigation headwinds.