WASHINGTON: A courtroom clash with US War Secretary Pete Hegseth is rapidly transforming Democratic Senator Mark Kelly into one of the most closely watched potential challengers to President Donald Trump ahead of the 2028 US presidential election.

A federal appeals court on Thursday appeared sceptical of Pentagon efforts to punish the retired astronaut and Navy officer over remarks urging US troops to refuse illegal orders, with judges sharply questioning whether the government’s case could stand constitutional scrutiny.

The case has taken on broader political significance in Washington, where Democrats are increasingly searching for figures capable of challenging Trump’s hardline political and security agenda.

Kelly, 62, is a former US Navy combat pilot who flew missions during the 1991 Gulf War before joining Nasa as an astronaut. He later commanded multiple space shuttle missions, including the final flight of Space Shuttle Endeavour in 2011.

He studied at the US Merchant Marine Academy and earned a master’s degree in aeronautical engineering from the Naval Postgraduate School.

Kelly has since built a political profile in Arizona and nationally, and is now widely discussed in Washington political circles as a possible Democratic presidential contender.

According to the magazine Politico, Kelly has raised more than $25 million in recent months, while also using the legal dispute to energise supporters and expand his national fundraising base.

The dispute began after Kelly and five other Democratic lawmakers — all with military or intelligence backgrounds — released a video last year warning against the politicisation of the US armed forces and reiterating that troops are not required to obey unlawful orders.

US President Donald Trump reacted angrily, accusing the lawmakers of “seditious behaviour,” while the Pentagon later initiated proceedings that could have reduced Kelly’s military retirement rank and benefits.

The case reached the US Court of Appeals for the District of Columbia Circuit this week, where a three-judge panel — Judges Karen LeCraft Henderson, Florence Pan, and Cornelia Pillard — questioned the government’s legal reasoning.

Judge Pillard said the central issue appeared to be mischaracterised by the government.

“The text of the video advises that service members have no obligation to obey unlawful orders,” she said. “Nobody in the video says service members have a duty to disobey lawful orders.”

Judges Pan and Henderson also pressed government lawyers on whether the Pentagon’s interpretation of the remarks could justify disciplinary action against a retired officer for expressing a constitutional view, while questioning whether any actual harm to military discipline had been demonstrated.

During the hearing, administration lawyers ultimately conceded that Kelly had not explicitly called for disobedience of lawful orders.

The Washington-based Cato Institute also supported Kelly, arguing that the Pentagon’s attempt to revoke his benefits amounted to unconstitutional retaliation for protected speech.

Outside the courtroom, Kelly said the case had implications far beyond his personal dispute with the Pentagon.

“I am not backing down,” he said. “After 25 years in the Navy, I have given too much to this country to be silenced by an administration that does not want to be held accountable.

“This was a day in court not just for me, but for millions of retired veterans and our First Amendment rights.”

The legal battle has significantly elevated Kelly’s national profile, with some Democrats already viewing him as a potential White House contender in a future election against Trump.

That political attention has also spilt into social media. Soon after the hearing, Kelly posted on Facebook, reiterating his stance and vowing to continue the legal fight. In the comments section under his post, reactions reflected both growing Democratic enthusiasm and Republican alarm.

Legal observers expect the dispute to eventually reach the US Supreme Court, potentially turning it into a major constitutional test of free speech, military authority, and the boundaries of political expression by former service members in the United States.