Thirty-eight Republican lawmakers are backing a lawsuit filed by Sen. Ron Johnson, R-Wis., challenging health insurance subsidies provided to lawmakers and their staffers who are required to obtain coverage under ObamaCare.

Johnson filed the lawsuit in January challenging a ruling by the Office of Personnel Management. The agency ruled that lawmakers and their staffs should continue to receive health care benefits covering about 75 percent of their premium costs after leaving the health insurance program for federal workers.

According to the lawsuit, the OPM ruling "does not treat members of Congress and their staffs like members' constituents. Instead, it puts them in a better position by providing them with a continuing tax-free subsidy."

On Tuesday, 38 Republicans filed an amicus brief accusing the Obama administration of attempting to "rewrite the Affordable Care Act."

"Courts must not shrink from their duty to enforce limits on executive power when necessary to protect the rights of individuals in actual cases and controversies. This case is a prime example," the 12 senators and 26 House members wrote.

The federal subsidies that members of Congress and their staff currently receive are in line with those paid by most private employers and are the same as other federal employees who are continuing in the federal plan. The lawsuit argues that they were not specifically detailed in the health care law and therefore are illegal.

Johnson told Fox News' Greta Van Susteren on Tuesday that the subsidy gives members of Congress and their staff "special treatment" that ordinary Americans forced to purchase health insurance under the law are unable to obtain.

"It’s unfair, it’s unfair treatment," Johnson said on "On the Record." "It’s special treatment, and by the way the president has no legal authority to change the law the way it did, and that’s really at the heart of this lawsuit is the doctrine and separation of powers and the fact that this president has exceeded his legal authority."

Johnson said some of his colleagues signed onto the brief despite receiving pushback from staff members who oppose the removal of the subsidy.

"These individuals have signed the suit, even though this will be contrary to their own financial best interest and the financial best interest of their staff, realize that what is at stake here is literally the constitutional balance, the constitutional framework of this nation," he said.

According to The Washington Post, co-signers of the brief include Sens. Ted Cruz, R-Tex.; John McCain, R-Ariz.; Saxby Chambliss, R-Ga.; Mark Kirk, R-Ill.; and Tim Scott, R-S.C. and Reps. Marsha Blackburn, R-Tenn.; Tom Cotton, R-Ark.; David Jolly, R-Fla.; Thomas Massie, R-Ky.; and Matt Salmon, R-Ariz.

The lawsuit could take months or even years to move through the legal system.

Democrats have criticized Republican efforts to kill the subsidy for lawmakers and their aides, saying it is petty and political. Johnson has also came under criticism from Rep. James Sensenbrenner, R-Wis., who called Johnson's lawsuit an "unfortunate political stunt."

Republicans have held more than 50 votes on changing or repealing the law over the past three years, knowing most would die in the Democratic Senate.

The Hill reported Tuesday that Rep. Kyrsten Sinema, D-Ariz., sent a letter to federal health officials earlier this month calling for an additional ObamaCare enrollment extension, citing constituent complaints about HealthCare.gov.

"These individuals should not be punished, either through a tax penalty or lack of coverage, because of ongoing technical or process challenges associated with the Affordable Care Act," Sinema said in the letter to the Centers for Medicare and Medicaid Services.

The Associated Press contributed to this report.