Louisville, Kentucky – In March, President Donald Trump travelled to the southern state of Kentucky to do an unusual thing in United States politics: campaign against a fellow Republican.

“Thomas Massie is a disaster for our party,” Trump told the crowd, his fists gripping either side of his podium.

He proceeded to excoriate Massie, a seven-term congressman known for his staunch conservatism and his willingness to buck the president's priorities.

“He’s disloyal to the Republican Party. He’s disloyal to the people of Kentucky," Trump said. "And most importantly, he’s disloyal to the United States of America, and he’s got to be voted out of office as soon as possible!”

Trump's rally took place at Verst Logistics, a packaging and shipment warehouse in Hebron, right in the heart of Kentucky’s 4th Congressional District.

Massie has represented the area since 2012. But on May 19, Republican voters in the district will cast their ballots in a primary election that may decide whether the congressman retains his seat.

This is no ordinary midterm race. Experts say the results of the Kentucky primary will show how far Trump can push the Republican Party and whether voters will stick with the president or their principles.

“I think this is a direct test of the president’s endorsement,” said Robert Kahne, a data scientist and Democratic leader from Louisville, Kentucky, who hosts a Kentucky politics podcast.

Kahne argues that Massie's main opponent, Ed Gallrein, has campaigned almost exclusively as Trump's chosen candidate. Trump endorsed Gallrein in October, before he had even entered the race.

"Basically, you have the strongest and most ardent Republican critic of Donald Trump on the ballot, against someone whose only identifying factor is being pro-Trump," Kahne said.