The Department of Defense is facing an onslaught of pushback from Republican governors who are questioning the Pentagon’s coronavirus vaccine mandate for troops.

The core battle at play is who has the ultimate authority over National Guard troops while they are under Title 32 of the U.S. Code, which details that the governor is the commander unless they are called up for federal duty, which would then put them under federal control.

Oklahoma Gov. Kevin Stitt was the first governor who raised concerns about the mandate in early November, and he argued that he, as the head of the state’s National Guard, had power over the National Guard. He filed a lawsuit on behalf of 16 Oklahoma Air National Guard members on Dec. 2, and a federal judge ruled in DOD's favor on Tuesday, according to the Washington Post.

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Six other GOP governors have followed Stitt’s path, while the Pentagon maintains that the order is lawful and necessary to ensure the readiness of the military.

Iowa Gov. Kim Reynolds, Alaska Gov. Mike Dunleavy, Mississippi Gov. Tate Reeves, Wyoming Gov. Mark Gordon, Nebraska Gov. Pete Ricketts, and most recently Texas Gov. Greg Abbott have spoken out against the mandate.

Five of those governors, excluding Abbott, wrote a letter to Defense Secretary Lloyd Austin on Dec. 14 urging him to reconsider the mandate. They argued, “directives dictating whether training in a Title 32 status can occur, setting punishment requirements for refusing to be COVID-19 vaccinated, and requiring separation from each state National Guard if unvaccinated are beyond your constitutional and statutory authority.”

Austin, in his correspondence to the Oklahoma governor, explained that his decision to deny the request “stems directly from my responsibility as the Secretary of Defense to promote the health, safety, and readiness of our military personnel.”

In a separate memo to the military services last month, Austin announced that National Guardsmen who refuse the vaccine will not be paid.

“No Department of Defense funding may be allocated for payment of duties performed under Title 32 for members of the National Guard who do not comply with Department of Defense COVID-19 vaccination requirements,” Austin wrote.

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The Defense Department has repeatedly maintained that personnel of any branch would face repercussions if they chose not to abide by the mandate or get an exemption. To date, each service branch has more than 90% of their active-duty forces vaccinated.

A Pentagon spokesperson declined to provide a comment and referred the Washington Examiner to DOD spokesman John Kirby's comments from a recent briefing.