Donald Trump Jr. headlines Montana Republican convention

Donald Trump Jr. headlines Montana Republican convention
© Greg Nash

Donald Trump Jr.Donald (Don) John TrumpFox News bigger than ever two years after Roger Ailes Kimberly Guilfoyle leaving Fox News to campaign with Donald Trump Jr.: report George Will charges that Trump colluded with Putin MORE is set to headline Friday's GOP convention in Montana, where he will seek to boost state auditor Matt Rosendale (R), who is running for Senate.

An invitation from Montana's Republican Party states that the president's oldest son is "energized" by Rosendale's campaign to defeat incumbent Sen. Jon TesterJonathan (Jon) TesterOvernight Defense: Trump inviting Putin to DC | Senate to vote Monday on VA pick | Graham open to US-Russia military coordination in Syria Senate to vote Monday on Trump's VA nominee On The Money: Trump rips Fed over rate hikes | Dems fume as consumer agency pick refuses to discuss border policy | Senate panel clears Trump IRS nominee MORE, a centrist Democrat.

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"Trump Jr. is coming back to Montana because he is energized by our U.S. Senate candidate, Matt Rosendale, and the opportunity we have to Take Out Tester in the fall! Trump Jr. knows how important it is to have a strong supporter like Matt Rosendale in the U.S. Senate to help our president Make America Great Again," the invitation reads.

Tester has emerged as a major target for the Trump administration after his rant against the GOP tax plan went viral last year and after he released a set of damning allegations that sunk Trump's nominee to run the Veterans Affairs Department, Dr. Ronny Jackson.

The pro-Trump super PAC America First Action has already targeted Tester over Jackson's failed nomination in ads ahead of November's midterm elections.

"In Montana, we value integrity and support our president," a narrator says in one ad from the super PAC. "But Jon Tester spread false information about a respected Navy admiral, helping D.C. Democrats derail President TrumpDonald John TrumpWSJ: Trump ignored advice to confront Putin over indictments Trump hotel charging Sean Spicer ,000 as book party venue Bernie Sanders: Trump 'so tough' on child separations but not on Putin MORE’s Veterans Affairs nominee."

"Jon Tester's been part of the D.C. swamp for far too long. He's betrayed our trust, our veterans and our president," it continues, calling Tester "disgraceful" and "dishonest."

The Republican chairman of the Senate Veterans' Affairs Committee has defended how Tester handled the release of the allegations against Jackson.

Trump Jr. previously tweeted his support for Rosendale in early June after Rosendale won a four-way GOP primary this month.

"Congrats to @MattForMontana for winning the GOP nomination for #MTSEN last night. Unlike his Chuck SchumerCharles (Chuck) Ellis SchumerData confirm that marijuana decriminalization is long overdue Pollster: Kavanaugh will get Dem votes Democrats slam Trump for considering Putin’s ’absurd’ request to question Americans MORE approved opponent, #MontanaMatt will back my dad’s America First agenda 100%," Trump Jr. tweeted on June 6.

A Gravis poll taken earlier this month showed Tester with a 8-point lead over Rosendale, twice the margin with which he won over Republican Rep. Dennis Rehberg in 2012.