Defense Secretary Pete Hegseth on Monday railed against one of President Donald Trump's chief Republican critics in Congress, stepping away from the Iran war and into a Kentucky congressional election that's testing the president's political clout.

Hegseth traveled to Kentucky to deliver his remarks, which were designed to undermine Rep. Thomas Massie on the eve of his primary in the state's 4th Congressional District. Hegseth said he was speaking "as a private citizen” — and not as a member of Trump's Cabinet.

And yet Hegseth referenced Trump over and over as he praised Massie's rival, former Navy SEAL Ed Gallrein.

"President Trump needs reinforcements, and that’s what war fighters do. They stand behind leaders and have their back," Hegseth said. “Too often, Thomas Massie has acted like his job is to stand apart from the movement that President Trump leads, instead of strengthening it. When President Trump needs backup, Massie wants to debate process.”

The defense secretary's political appearance, which represents an extraordinary break from tradition with the nation at war, comes as Trump looks to demonstrate his continued dominance of the Republican Party, even as he struggles to address heightened concerns about the economy and Iran war less than six months before the midterm elections.

Massie's primary, perhaps more than others this midterm season, is testing Trump's clout. The president's allies have poured tens of millions of dollars into the campaign against Massie in what has become the most expensive congressional primary in U.S. history.

Massie, a rare Republican congressman who has openly clashed with Trump, has criticized the war in Iran and aggressively pushed for the release of the Jeffrey Epstein files.

“They’re desperate. That’s why they’re sending the secretary of war to my district,” Massie said Sunday on ABC's “This Week.” “That’s why the president’s losing sleep and tweeting about this.”

Ahead of Hegseth's appearance in Kentucky, the Pentagon issue a statement in response to criticism that the defense secretary was breaking the law by using taxpayer dollars for political purposes.

“Secretary Hegseth is attending this event in his personal capacity. No taxpayer dollars will be used to facilitate his visit," said Chief Pentagon Spokesman Sean Parnell. "His participation has been thoroughly vetted and cleared by lawyers, including the Department of War Office of General Counsel, and does not violate the Hatch Act or any other applicable federal statute.”

___ Associated Press writer Konstantin Toropin in Washington contributed to this report.