The Pentagon said they are considering all disciplinary options against Senator Mark Kelly.

Senate Democrats on the chamber's Armed Services Committee called the Pentagon's investigation into Senator Mark Kelly, D-Arizona, "an outright, brazen abuse of power" in a letter sent to Secretary of the Navy John Phelan on Dec. 9.

"We believe this 'review' along with the Department of Defense's social media post announcing a 'thorough review' of Senator Kelly's actions ... amount to a purely political exercise seeking to threaten legitimate and lawful actions by a duly elected Senator, and politicize our military justice system," according to the letter provided to USA TODAY.

The Pentagon announced on Nov. 24 that it was investigating Kelly for "serious allegations of misconduct." The announcement came after Kelly joined five other Democratic lawmakers who released a video on Nov. 18 telling service members they have the right not to obey orders they believe are illegal.

The letter notes that while military retirees generally remain subject to the Uniform Code of Military Justice, it calls a potential disciplinary action for "restating the law as articulated in the UCMJ" an " incredibly dangerous precedent."

A Nov. 25 memo posted by the Department of Defense, under the moniker of the Department of War, states that Phelan is to brief Defense Secretary Pete Hegseth on the investigation "no later" than Dec. 10.

USA TODAY reached out to Phelan's office for a response and did not receive an immediate response. A representative for the Republicans on the committee also did not immediately respond to a request for comment.

Senate Democrats attack impartiality of Kelly investigation

The letter points to social media posts and public comments made by President Donald Trump and Hegseth to call the investigation "baseless and patently political."

"While the Department's official statement suggests that the 'review' will be conducted 'ensuring due process and impartiality,' the President and his subordinates have made fair proceedings impossible," the letter reads.

President Donald Trump accused the lawmakers in the video of "SEDITIOUS BEHAVIOR, punishable by DEATH!" in a Nov. 20 post to Truth Social but later claimed that he wasn't "threatening death" on them in a Nov. 21 interview on Fox News Radio.

The letter is signed by 12 of the 13 Democrats on the committee, with Kelly being the lone member of the minority who didn't sign. USA TODAY reached out to Kelly for comment and did not receive an immediate response.

Kelly predicts Hegseth will 'take a hike' on investigation

Kelly has taken a defiant stance following the announcement of the investigation.

He scoffed at the possibility of facing a court-martial stemming from the investigation as he ridiculed President Donald Trump and Hegseth as unserious people during a Dec. 5 town hall event in Tucson.

The former Navy combat pilot told the 200 people on hand that he expects Hegseth "is just going to take a hike" on a court-martial.

"It's certainly unconstitutional," Kelly said. "They're not serious people and I'm not backing down."

Kelly accused Trump of attempting to silence critical voices in the military and the country at a Dec. 1 press conference on Capitol Hill.

"They're trying to send a message to retired service members, to government employees, to members of the military, to elected officials, and to all Americans who are thinking about speaking up, you better keep your mouth shut, or else," Kelly said.

Legal and military experts previously told USA TODAY that a case against Kelly faces major hurdles and would likely be "dead on arrival" in military courts.

Contributing: Joey Garrison - USA TODAY

Senate Democrats call Kelly investigation 'brazen abuse of power'