US Senator Mark Kelly has sued Defence Secretary Pete Hegseth, arguing that the Pentagon illegally tried to demote him in retaliation for criticising the Trump administration.

Kelly, a former Navy captain, attracted the ire of Hegseth and President Donald Trump after he filmed a video that told military personnel they could refuse unlawful orders.

Trump accused Kelly of 'sedition,' and Hegseth said he would open proceedings to lower Kelly's retirement grade, which could lead to a pay cut.

The case alleges these actions violated Kelly's right to free speech and due process, and did not adhere to proper military procedure. The Pentagon and White House did not immediately comment.

Kelly also had a significant career as an astronaut before being elected to the US Senate for Arizona in 2020. He founded a gun safety group with his wife, Gabby Giffords, a former Congresswoman who survived a shooting in 2011.

As a centrist Democrat, Kelly critiqued Trump’s deployment of National Guard troops to urban areas, presenting a contrasting narrative to the administration’s justification of public safety measures.

In November, Kelly and several Democrats recorded a video stating, 'Our laws are clear: you can refuse illegal orders,' prompting Trump's harsh reactions on social media.

Hegseth subsequently announced he would initiate retirement-grade proceedings against Kelly, labeling his comments 'seditious in nature.' Kelly's lawsuit contends Hegseth's actions infringe on his speech rights and were legally unfounded.