Donald Trump Jr. headlined Montana's Republican Party convention on Friday, stumping for state auditor Matt Rosendale, a former state lawmaker now running for a U.S. Senate seat.

The seat is currently held by Sen. John Tester, a Democrat who drew President Donald Trump's ire earlier this year by criticizing Dr. Ronny Jackson, the former presidential physician whom the president had nominated to head the Department of Veterans Affairs.

Jackson ultimately withdrew his nomination after facing allegations of misconduct and mismanagment. Soon after, he also ceased being the president's physician.

On Friday, the president's son told the convention gathering in Billings that Rosendale was the best choice to replace Tester.

"I'm the son of a billionaire from New York City and I have much more of a Montana platform than the senator, the senior senator from this state. That doesn't make much sense," Trump Jr. told the crowd of several hundred Republicans.

"I'm the son of a billionaire from New York City and I have much more of a Montana platform than the senator, the senior senator from this state. That doesn't make much sense." — Donald Trump Jr., campaigning for Matt Rosendale, a U.S. Senate candidate in Montana

Rosendale prevailed June 5 in a four-way primary election after outside political groups spent several million dollars promoting his candidacy and attacking his Republican opponents.

In a Twitter message earlier this month, Trump Jr. wrote that Rosendale would back President Donald Trump's “America First agenda 100%.”

In 2016, candidate Donald Trump captured Montana by 20 percentage points.

The president’s son was joined onstage by U.S. Sen. Steve Daines, U.S. Rep. Greg Gianforte and Rosendale.

Tester has not backed away from the actions he took against Jackson and said in a recent interview that he would welcome a trip by the president to Montana.

"I did my job as a U.S. senator. We vetted him (Jackson) we asked questions, we didn't get any answers and he pulled out," Tester said. "I hope (President Trump) comes to Montana. I really do. I hope he comes and looks at some of the veterans clinics we have, here some of the infrastructure needs and some of the border security challenges we have on our Northern border."

Sen. Johnny Isakson, R-Ga., chairman of the Senate Veterans Affairs Committee, has defended how Tester handled the release of the allegations against former VA nominee Jackson, according to the Hill.

The Associated Press contributed to this report.