(The Center Square) – All military members will be required to take the COVID-19 vaccine, the Pentagon announced Monday, but some members of Congress have already mobilized an effort to overturn the mandate.

The press secretary for the Department of Defense, John Kirby, said Monday that Secretary Lloyd Austin would require the vaccine for service members. That decision came just hours after news broke that the Food and Drug Administration had given full approval to Pfizer’s COVID-19 vaccine.

Austin pledged in a memo earlier this month to make the vaccine mandatory by mid-September or when the FDA approved it, whichever came first.

“So, over the last week, I have consulted closely with the Chairman of the Joint Chiefs of Staff, the Secretaries of the Military Departments, the Service Chiefs, and medical professionals,” Austin wrote. “I appreciate greatly the advice and counsel they provided.”

President Joe Biden predicted the Pfizer approval earlier this month and preemptively endorsed the Pentagon’s mandate decision.

“By way of expectation, public reporting suggests the Pfizer-BioNTech vaccine could achieve full FDA licensure early next month,” Biden said. “The intervening few weeks will be spent preparing for this transition. I have every confidence that Service leadership and your commanders will implement this new vaccination program with professionalism, skill, and compassion. We will have more to say about this as implementation plans are fully developed.”

A group of Republicans have already begun work to overturn the requirement. U.S. Rep. Thomas Massie, R-Ky., raised the alarm earlier this year about the fallout of this decision. He said in July several service members had told him to expect mass resignations if the vaccine is mandated. Whether those claims prove true remains to be seen.

“According to the GAO and congressional testimony, there were similar results (departures) when the military mandated the anthrax vaccine,” Massie said.

Massie introduced a bill earlier this year “to prohibit any requirement that a member of the Armed Forces receive a vaccination against COVID-19.”

So far, Massie’s bill has 31 cosponsors, all Republican. It bans the vaccine requirement regardless of FDA approval.

“This bill prohibits the use of federal funds to require a member of the Armed Forces to receive a COVID-19 vaccination,” the bill’s summary reads. “The bill also prohibits adverse action (e.g., punishment) being taken against a member of the Armed Forces because the member refuses to receive a COVID-19 vaccination.”

Biden enacted a mandate in July that federal employees receive the vaccine, or face strenuous requirements, including weekly testing. Meanwhile, several large corporations, including WalMart, Google, United Airlines, and others have put mandates in place as well. With the FDA approval of Pfizer’s vaccine, that number is expected to rise.

Meanwhile, Sen. Ron Johnson, R-Wis., raised questions about the FDA approval, arguing it was “rushed” to make way for vaccine mandates. Other Republicans in the Senate, though, took the opposite tact, encouraging Americans to get vaccinated.

Health officials have rebuffed Johnson’s claims, but he sent a letter to Janet Woodcock, acting commissioner of the Food and Drug Administration, Francis Collins, director of the National Institutes of Health, and Rochelle Walensky, director of the U.S. Centers for Disease Control. He argues in the letter that federal officials skipped steps, such as not holding a formal advisory committee meeting on the Pfizer vaccine, where independent analysis is provided to federal health officials.

“I see no need to rush the FDA approval process for any of the three Covid-19 vaccines,” Sen. Johnson wrote. “Expediting the process appears to only serve the political purpose of imposing and enforcing vaccine mandates. The observational phases of FDA approval take time, because there is no substitute for time in detecting and determining possible long-term harm. Additionally, we are already experiencing a severe health care worker shortage. Frontline doctors and nurses that are contacting me are expressing grave concerns about vaccine mandates which will only exacerbate the shortage.”