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The Pentagon on Monday said it was investigating Democratic Sen. Mark Kelly (Ariz.) — a retired naval officer who recently clashed with President Trump — over "serious allegations of misconduct." Why it matters: Kelly is part of a group of Democratic lawmakers who last week posted a video calling on military service members to disobey unlawful orders. Trump demanded that the lawmakers be "ARRESTED and PUT ON TRIAL." A statement from the Pentagon on Monday did not specify the nature of the allegations but said all service members have a "legal obligation" to "obey lawful orders and that orders are presumed to be lawful." A representative for Kelly did not immediately respond to a request for comment. The big picture: Democrats rallied around Kelly and colleagues last week after Trump posted on social media that the video was "SEDITIOUS BEHAVIOR, punishable by DEATH." "Your continued threats will not intimidate me or keep me from doing my job, which includes oversight of the executive branch," Kelly said on X Sunday. "Stop the threats of execution, hanging, and sending a mob before someone gets hurt." Senate Minority Leader Chuck Schumer (D-N.Y.) said last week: "I've never seen a president or heard of a president who has done more to encourage political violence." Schumer also said he has asked the U.S. Capitol Police to give Kelly and Sen. Elissa Slotkin (D-Mich.) "special protection." Catch up quick: In the video, Kelly, along with Slotkin and Reps. Jason Crow (D-Colo.), Chris Deluzio (D-Pa.), Maggie Goodlander (D-N.H.) and Chrissy Houlahan (D-Pa.), urged service members to defy unlawful orders. "This administration is pitting our uniformed military and intelligence community professionals against American citizens," the Democrats said. "Our laws are clear: You can refuse illegal orders." White House press secretary Karoline Leavitt said at a press briefing last week that Trump's social media posts about the video were not meant as threats. Trump suggested in an interview that aired Friday that the Defense Department was investigating the six Democratic lawmakers. The other side: Secretary of Defense Pete Hegseth said Monday that the video from Democrats "was despicable, reckless, and false." "Encouraging our warriors to ignore the orders of their Commanders undermines every aspect of 'good order and discipline,'" Hegseth said in a post on X . The Pentagon post points to a federal law that bars intentionally urging U.S. troops to disobey orders or undermine military discipline, carrying penalties of up to 10 years in prison. Editor's note: This story has been updated with additional reporting.