Republican governors are threatening to fight US President Joe Biden's new vaccine mandate covering big companies, healthcare workers and federal employees.
“Have at it,” Mr Biden said on Friday when asked about the threat of lawsuits.
The mandate, which the White House says would cover 100 million US workers and applies to about two thirds of all US employees, is being written in part by the US Department of Labour's Occupational Safety and Health Administration (OSHA).
But some Republicans have already declared the move to be an unconstitutional overreach by the federal government.
Governors Kristi Noem of North Dakota, Henry McMaster of South Carolina and Brian Kemp of Georgia are threatening to sue.
Texas Governor Greg Abbott said on Thursday night that his state “is already working to halt this power grab".
The Republican National Convention said in a statement that it also intends to sue the Biden administration, but not until the executive order goes into effect.
The New Civil Liberties Alliance, a non-profit funded by the Charles Koch Foundation, claimed Mr Biden's plan “vastly exceeds the powers the United States constitution allots the executive branch".
Mr Biden's comments came during a tour of a school in Washington, where he outlined his support for schools enacting measures to prevent the spread of Covid-19.
During his tour, Mr Biden lambasted Republican governors for fighting mask mandates.
“I am so disappointed that particularly some Republican governors have been so cavalier with the health of these kids,” Mr Biden said. “So cavalier with the health of their communities. We’re playing for real here. This isn’t a game.”
Republican governors including Ron DeSantis of Florida and Mr Abbott have tried to prohibit schools in their states from requiring masks, calling it a personal decision for parents.
The president said that both science and public opinion are on his side, noting polls that show strong support for mask mandates in schools.
“One of the lessons that I hope our students can unlearn is that politics doesn’t have to be this way,” Mr Biden said. “It’s not how we are as a nation. And it’s not how we beat every other crisis in our history. The vast majority of the American people know we have to do these things.”
In his speech on Thursday, Mr Biden expressed his frustration at Americans who are still not vaccinated.
“We’ve been patient, but our patience is wearing thin,” Mr Biden said. “Your refusal has cost all of us.”
The disease has killed more than 654,000 Americans, and deaths and hospital admissions have been rising sharply as the easily transmissible Delta variant of the virus spreads. The vast majority of those cases and deaths have been among the unvaccinated.