Senator Mark Kelly is suing Defense Secretary Pete Hegseth, accusing him of conducting an “unconstitutional crusade” against Mr. Kelly because of a video message the senator directed to members of the United States military about following illegal orders.

The lawsuit, filed Monday in the United States District Court for the District of Columbia, also names the Department of Defense as a defendant for its effort to downgrade Mr. Kelly’s military retirement rank.

In November, Mr. Kelly appeared in a video with five other Democratic members of Congress telling members of the military to disregard unlawful orders. The video was released shortly after President Trump discussed sending troops to Chicago to quell anti-ICE protests.

In the video, the lawmakers — all of whom served in the military or as members of the intelligence community — said they believed servicemembers were under “enormous stress and pressure right now.”

Mr. Trump and members of his administration lashed out at Mr. Kelly after its release.

Mr. Trump, in a social media post, called the video “seditious behavior at the highest level.”

“Each one of these traitors to our Country should be ARRESTED AND PUT ON TRIAL,” the president wrote. “Their words cannot be allowed to stand — We won’t have a Country anymore!!! An example MUST BE SET.”

The “Official White House Rapid Response account” reposted an interview with Stephen Miller, who labeled the statements an “insurrection” and a “general call for rebellion.”

Mr. Hegseth then ordered a probe of Mr. Kelly’s statements.

Last week, Mr. Hegseth issued a censure against Mr. Kelly and the Navy started the process to reduce the 25-year veteran’s rank and retirement pay. Mr. Kelly retired from the Navy in 2011.

“As a retired Navy Captain who is still receiving a military pension, Captain Kelly knows he is still accountable to military justice,” Mr. Hegseth wrote on X last week. “And the Department of War — and the American people — expect justice.”

Mr. Kelly said he has faced “extreme rhetoric and punitive retribution” over his statements. He says he can not be punished in retirement for a political statement he made as a current member of Congress.

In an X post announcing his lawsuit, Mr. Kelly said, “I filed a lawsuit against the Secretary of Defense because there are few things as important as standing up for the rights of the very Americans who fought to defend our freedoms.”

Mr. Kelly established a legal fund last month to help defend himself against federal investigations launched by the Trump administration.

The senator is asking a judge to block his censure. The suit also claims the actions being taken against him violates the separation of powers between the legislative and executive branches. Mr. Kelly’s suit also accuses the military of violating his due process and First Amendment rights.

“We are aware of the litigation,” a spokesman with the Pentagon said in an emailed statement to the New York Sun. “However, as a matter of policy, the Department does not comment on ongoing litigation.”