Donald Trump Jr., President Trump's eldest son, is on the hunt in the state of Montana, setting his focus on ensuring that Sen. Jon Tester, a Democrat who has served in the U.S. Senate for more than 11 years, doesn't get re-elected this year.

He has visited the Northwestern state to campaign for Republican candidates, including last month when he spoke at the Montana Republican Party's annual convention in Billings. There he appeared alongside Sen. Steve Daines, Rep. Greg Gianforte, and State Auditor Matt Rosendale, who is challenging Tester this November and recently won the GOP primary.

"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. said at the event, where he also panned Tester as being "all for illegal immigration, all for sanctuary cities" and for "writing and proposing legislation against the second amendment."

Tester found himself in the cross hairs of the president earlier this year when he contributed to the derailment of Trump's nomination of White House physician Ronny Jackson to be Veteran Affairs secretary, sharing stories from anonymous sources who claimed Jackson had a drinking problem and was not judicious in prescribing sleep medication or opioid painkillers.

In a flurry a tweets in April, Trump called Tester's actions "Very dishonest and sick!" and called for him to resign or lose his re-election bid — part of an offensive that gave Republicans a sense that they had a new opening against a Democratic incumbent.

But Tester appeared to be undeterred, saying, "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," according to the Associated Press.

Trump's attacks against Tester have dissipated, but the offensive has been renewed by his son, who tweeted in June, after Rosendale won the GOP nomination: "I look forward to visiting Montana early and often to help Matt defeat #TwoFacedTester in Nov!"

After appearing at the GOP convention later that month, Trump Jr. and new flame Kimberly Guilfoyle of Fox News spent the weekend fishing in the Stillwater River. Trump Jr. has also been hunting in the state. In April 2017 he spent a weekend with then-candidate Gianforte hunting prairie dogs.

Trump Jr. kept the hits against Tester coming with an op-ed published Tuesday in the Great Falls Tribune, one of the state's largest newspapers. Touting his father's electoral appeal in the state — he won Montana by 20 points in 2016 — the headline of Trump Jr.'s blared: "Jon Tester is no partner of President Trump."

While Tester has aired an ad boasting of his ability to work with Trump, Trump Jr. declared the senator only "pretends he’s willing to work with President Trump to Make America Great Again, but his Chuck Schumer-approved, liberal record proves otherwise." Calling the two-term senator a "liberal lapdog" to Senate Minority Leader Chuck Schumer, D-N.Y., Trump Jr. claimed Tester has a "voting record [that] is nearly as radical as liberal extremists like Elizabeth Warren."

"Tester has voted no on tax cuts, no on repealing and replacing Obamacare, no on cracking down on sanctuary cities, no on repealing stifling regulations, and has continually supported gun control measures that would restrict our precious 2nd Amendment rights — all of which runs directly counter to my father’s America First agenda," Trump Jr. wrote.

Though he was just in the state, Trump Jr. is already slated to make a comeback in the next few days. The Great Falls Tribune reported Tuesday that Trump Jr. will join his father at a rally in Great Falls on Thursday, one day after the Fourth of July holiday.

Trump Jr.'s preferred candidate still appears to have an uphill battle, despite the high-profile support.

A Gavis poll taken last month showed Tester with an 8 percentage point lead over Rosendale. The margin is notable as Tester beat his GOP opponent Denny Rehberg by just over 4 points back in 2012, according to Ballotpedia.