The conservative Club for Growth is out with an early endorsement of Kevin Nicholson's Senate primary bid in Wisconsin.

In a statement Tuesday morning, the conservative group praised Nicholson, a Marine Corps veteran and businessman, praising him as a "strong economic conservative" and a "political outsider."

Nicholson is the only candidate currently declared for the 2018 GOP primary.

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“Club for Growth PAC is thrilled to announce support for Kevin Nicholson and his campaign for U.S. Senate,” wrote Club for Growth PAC's President David McIntosh.

"Kevin is not just a political outsider, he’s a strong economic conservative," he added, "who understands that free markets and limited government are the engines of economic growth."

Club for Growth's endorsement is the second major endorsement for Nicholson, who announced his bid last month and was endorsed by former U.S. Ambassador John Bolton.

Businessman Eric Hovde, seen as a likely challenger, recently sold off a large amount of stock as he weighs jumping into the race. Hovde most recently ran in 2012, when he came in second place over several established politicians and nearly overtook the state's former Gov. Tommy Thompson in the Republican Senate primary.

State Sen. Leah Vukmir is also expected to announce her candidacy.

The winner of the GOP primary would take on Sen. Tammy Baldwin Tammy Suzanne BaldwinKeep teachers in the classroom Cher raised million for Biden campaign at LGBTQ-themed fundraiser Democrats seek balance in backing protests, condemning violence MORE (D-Wis.), who is completing her first term.

President Trump won Wisconsin in 2016, giving Republicans hope they can defeat Baldwin. The president's approval ratings could be a problem, however. Trump hit a new low in Gallup's daily tracking poll in Monday of 34 percent, with 61 percent disapproval.

Republicans hold just a two-seat majority in the Senate, but only have two somewhat vulnerable members up for reelection in 2018: Sen. Dean Heller Dean Arthur HellerOn The Trail: Democrats plan to hammer Trump on Social Security, Medicare Lobbying World Democrats spend big to put Senate in play MORE (Nev.), who is considered the most endangered Senate Republican, and Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden Maybe they just don't like cowboys: The president is successful, some just don't like his style MORE (Ariz.), who may face a Republican challenger backed by Trump.

Democrats, in contrast, are defending dozens of seats, including 10 in states won by Trump.

Nicholson, a veteran of the 2007 Iraq troop surge, announced his Senate run a month ago, telling National Review that Baldwin and former President Obama were responsible for throwing away U.S. progress in the fight against terror in the Middle East.

“Obama and leaders like Tammy Baldwin threw our progress in Iraq away,” Nicholson told National Review. “Irresponsibly pulling out in the way they did has led to problems like what we see with ISIS today.”

“I know for a fact that Obama’s drawdown encouraged the Taliban and other enemies to kill Americans,” he said.