PHOENIX >> Arizona Attorney General Mark Brnovich filed the first lawsuit today to block President Joe Biden’s new coronavirus vaccine requirements that could affect 100 million workers.

Brnovich, who is running in a crowded Republican primary for U.S. Senate, claims Biden is illegally treating U.S. citizens and legal immigrants differently from people caught crossing the border illegally, many of whom are offered vaccines but not required to accept them.

Brnovich’s lawsuit is the first of many expected challenges to Biden’s proposed vaccine rules. But while many details about the rules remain unknown, Biden appears to be on firm legal ground to issue the directive in the name of protecting employee safety, according to several experts interviewed by The Associated Press.

Biden said last week that his administration will require employers with at least 100 workers to ensure everyone is either vaccinated or tested weekly for COVID-19, affecting about 80 million workers. The roughly 17 million workers at health facilities that receive federal Medicare or Medicaid also will have to be fully vaccinated. So will employees of the executive branch and contractors who do business with the federal government — with no option to test out. That covers several million more workers.

“The Biden administration has no authority under the Constitution to mandate COVID-19 vaccines, period,” Brnovich said. “This is a heavy-handed attempt by the federal government that shows big government at its worst, not its best.”

Brnovich told reporters in a conference call that Biden is overstepping his constitutional powers and argued that the Congress has delegated too much power to the president. But his lawsuit is focused on what he claims is the differential treatment of U.S. citizens and those living in the country illegally.

“It is not a sensible argument,” said Paul Bender, a constitutional law professor at Arizona State University. “It’s worse than nonsensical. It’s really laughable.”

The lawsuit is based on “a lie” that Biden’s vaccine rules will exclude immigrants, Bender said. If immigrants living illegally in the country are working without authorization at an employer covered by Biden’s vaccine rules, they will have to follow them.

Even if a court were to buy Brnovich’s argument, the most likely outcome would be to impose the same vaccine and testing requirements for immigrants, not to eliminate them for citizens, Bender said.

Picking fights with the Biden administration could help Brnovich as he faces well-funded opponents in a tough primary to challenge Democratic U.S. Sen. Mark Kelly, one of the GOP’s top pickup targets in 2022.

The Department of Labor has not issued Biden’s rules for them to be directly challenged in court. The White House is gearing up for a wave of lawsuits from Republican officeholders and potentially affected employers that oppose the rules. White House press secretary Jen Psaki outlined the legal justification Friday, saying the Department of Labor has an obligation to keep workplaces safe.

“The law basically requires the Department of Labor take action when it finds grave risk to workers,” Psaki said. “And certainly a pandemic that killed more than 600,000 people qualifies as grave risk to workers.”

Brnovich filed his lawsuit in U.S. District Court in Phoenix.