WASHINGTON — Democratic Sen. Mark Kelly sued the Pentagon on Monday over attempts to punish him for his warnings about illegal orders, claiming the Trump administration trampled on his constitutional rights to free speech.

Kelly, a former U.S. Navy pilot who represents Arizona, is seeking to block his censure last week from Defense Secretary Pete Hegseth, who said he censured Kelly over his participation in a video that called on troops to resist unlawful orders.

Hegseth said the Jan. 5 censure — by itself simply a formal letter with little practical consequence — was “a necessary process step” to proceedings that could result in a demotion from Kelly’s retired rank of captain and subsequent reduction in retirement pay.

Kelly asked the federal court in Washington, D.C., to rule that the censure letter, the proceedings about his rank and any other punishments against him are “unlawful and unconstitutional.”

“The First Amendment forbids the government and its officials from punishing disfavored expression or retaliating against protected speech,” his lawsuit says. “That prohibition applies with particular force to legislators speaking on matters of public policy.”

The case was assigned to U.S. District Judge Leon, who was nominated to the bench by Republican President George W. Bush.

The legal action of a sitting U.S. senator suing the defense secretary is rare, if not jarring, and the latest effort from members of Congress to push back against what they see as an out-of-control executive branch.

The censure stemmed from Kelly’s participation in a video in November with five other Democratic lawmakers — all veterans of the armed services and intelligence community — in which they called on troops to uphold the Constitution and not to follow the Trump administration’s military directives if they were unlawful.

The 90-second video was first posted on a social media account belonging to Sen. Elissa Slotkin. Reps. Jason Crow, Chris Deluzio, Maggie Goodlander and Chrissy Houlahan also appeared in the video.

Republican President Donald Trump accused the lawmakers of sedition “punishable by DEATH” in a social media post days later.