Montana state Auditor Matt Rosendale said Monday he would challenge Sen. Jon Tester Jonathan (Jon) TesterPence seeks to boost Daines in critical Montana Senate race This World Suicide Prevention Day, let's recommit to protecting the lives of our veterans Filibuster fight looms if Democrats retake Senate MORE (D-Mont.), giving Republicans a top recruit in their effort to win a red-state Senate seat.

Rosendale will enter a crowded GOP primary field but will be the only Republican in the race to hold a statewide office.

Tester, who holds one of the 25 seats the Senate Democratic Caucus is defending next year, is seen as vulnerable. He won his 2012 reelection by about 4 percentage points.

Rosendale announced his bid on Monday with a short video blasting Washington Democrats and touting his conservative chops.

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The video specifically touts Rosendale as a would-be ally to President Trump and Vice President Pence, making clear he would support the administration in a state Trump won by about 20 points in November.

"There's a shadow over Montana — an intrusive federal government run by insiders, liberals and big spenders. They are everything that is wrong with Washington," the narrator of the ad says. "President Trump and Mike Pence Michael (Mike) Richard PenceGardner signals support for taking up Supreme Court nominee this year Biden leads Trump by 12 points among Catholic voters: poll GOP brushes back charges of hypocrisy in Supreme Court fight MORE are fighting back, but they need help."

The ad goes on to laud Rosendale as a conservative "rancher, businessman [and] proven leader."

"He'll stand up to Washington and get them out of our backyard. Matt Rosendale means business," the ad says in closing.

Rosendale joins a field of Republican challengers that includes state Sen. Al Olszewski and two businessmen, Ronald Murray and Troy Downing. Initially, Republicans had hoped that former Rep. Ryan Zinke (R-Mont.) would run against Tester, but Zinke accepted Trump's invitation to lead the Interior Department.

After Zinke, Republicans had pushed state Attorney General Tim Fox to consider a bid. But he announced last month that he wanted to remain in his current position.

Rosendale enters the race for the GOP nomination in the driver's seat. He previously served in the state's legislature, reaching the rank of Senate majority leader for the 2015-2016 term.

The Republican field hasn't raised much money to take on Tester, who reported had $4.7 million in the bank by the end of June.

Downing leads the money race with $155,000 in his bank account after a $100,000 personal loan. Olszewski has about $31,000 in his campaign account, while Murray hasn't yet filed his report.

Rosendale won't have to file a fundraising report until October, but he has an active account left over from his 2014 bid for the House, in which he lost to Zinke. He has about $5,000 in his campaign account but still remains about $217,000 in debt from that race.