Republican senators are tepidly coming to Senator Mark Kelly’s defense after President Trump and his administration threatened the retired Navy captain with a court martial and an FBI investigation. After Mr. Kelly and five other Democrats urged servicemembers to ignore illegal orders, Mr. Trump began saying that the lawmakers should be jailed. 

Mr. Kelly, an Arizona Democrat who flew combat missions in the Persian Gulf and served on four NASA flights as an astronaut, was singled out by the Defense Department for a potential court martial because of the respective retirement statuses of the other Democrats. Mr. Kelly says it is just an attempt to “intimidate” him. 

The four House members who appeared in the video urging servicemembers to ignore illegal orders announced Tuesday that the FBI is also seeking interviews with them and Mr. Kelly, as well as Senator Elissa Slotkin, for their message. 

After days of Mr. Trump and his allies attacking Mr. Kelly, two Republican senators came to his defense on Tuesday, though those GOP lawmakers did not mention Mr. Trump. 

“Senator Kelly valiantly served our country as an aviator in the U.S. Navy before later completing four space shuttle missions as a NASA astronaut,” Senator Lisa Murkowski, the Alaska Republican, said in an X post on Tuesday. “To accuse him and other lawmakers of treason and sedition for rightfully pointing out that servicemembers can refuse illegal orders is reckless and flat-out wrong.” 

“The Department of Defense and FBI surely have more important priorities than this frivolous investigation,” Ms. Murkowski said. 

Senator John Curtis, a more moderate freshman from Utah, sent out a more vague defense, though a defense nonetheless. In an X post, Mr. Curtis praised Mr. Kelly for his service. 

“As a colleague, I respect Mark Kelly and value his friendship. I know him as someone whose career has been defined by service,” Mr. Curtis wrote on Tuesday night. “His record as a combat naval aviator and NASA astronaut reflects his example of the discipline and dedication that are important for success.”

Democrats and some civil libertarians had more strident defenses of Mr. Kelly and his actions. 

Senator Ruben Gallego, Mr. Kelly’s Arizona counterpart, used coarse language in a video to criticize administration figures. “These guys are trying to say that they’re not acting like fascists,” Mr. Gallego said. A former Republican congressman-turned-Libertarian, Justin Amash, went so far as to call for Mr. Hegseth’s impeachment on Tuesday, saying that he was guilty of trying to use the military justice system to punish Mr. Kelly’s protected speech. 

“Even if it were politically motivated, it would not be unlawful to make such a video. [Mr. Hegseth’s] effort to mislead millions of Americans about the Constitution, the freedom of speech, and the basic operation of law is abhorrent,” Mr. Amash wrote in an X post on Tuesday.