MADISON, Wis. — The outcome of a hotly-contested “insider versus outsider” Republican primary in Wisconsin could potentially decide which party holds the Senate after November.

The outsider is Kevin Nicholson — a young, decorated veteran-turned-businessman who walked away from the Democratic Party during his service in the U.S. Marine Corps. The insider is state lawmaker Leah Vukmir — a nurse-turned politician with almost sixteen years in the Wisconsin legislature. The winner of Tuesday’s primary will face incumbent Sen. Tammy Baldwin in what Democrats are treating as a vulnerable seat this year. Baldwin has a large war chest, but her attempt to moderate away from her hyper-liberal, Madison-centric former congressional district has been marred by several controversies that offer the eventual GOP opponent plenty of ammunition.

Republicans tried to seize this seat in 2012 when then-Democratic Sen. Herb Kohl retired. As it turned out, a bad year for Republicans nationally compounded by an aging GOP nominee who had spent his entire adult life in elected or appointed office gave Baldwin the opening she needed.

Nicholson knows this history, and so his biography as a problem-solving businessman and veteran who is running for office for the first time represents a different formulate for winning the seat. Politico — hardly a friend of conservatives — touted him as a Republican “dream candidate” with one small caveat: he used to be a Democrat.

His Democratic past isn’t something Nicholson runs from. Repeatedly he opens his stump speeches with a self-deprecating comment about his youthful work for College Democrats, the campus organizing arm of the DNC. Growing up just north of Milwaukee, Nicholson was raised in a household of Democrats. His parents have already donated to Baldwin’s re-election effort.

His political conversion story starts when he joined the Marine Corps after college, volunteering for combat tours in Iraq and Afghanistan. It was while serving in the Middle East, he says, that he realized his worldview wasn’t holding up under the harsh realities of life and war. Marriage, fatherhood and a coming to faith rounded out his maturation. The pro-choice college Democrat became a pro-life, free-market conservative. Conversion stories are what conservatives repeatedly tout as evidence of the power of their first principles. In Nicholson, they have a champion.

Despite being opposed by the Republican Party infrastructure, which determined in a convention vote that it wanted to back the state legislator, Nicholson has run a formidable campaign. He has consistently raised more cash than his primary opponent, earned all of the high-profile national endorsements, and built a grassroots base that includes 10,000 donors across all of Wisconsin’s 72 counties.

Groups such as the Senate Conservatives Fund, Club for Growth, Great America PAC and Madison Project have all backed Nicholson with spending. National Security Advisor John Bolton has endorsed him, as have conservative rock stars in the Senate, Ted Cruz and Mike Lee. That makes Nicholson the only candidate in the GOP primary to be endorsed by Trump administration officials and sitting U.S. senators.

Leah Vukmir, the state lawmaker challenging Nicholson, can tout her support for various state-level reforms that have put Wisconsin in national headlines in recent years. On the stump she rarely goes into details about those reforms, however, choosing to generically say she has supported the “Wisconsin way” and wants to bring that same approach to Washington. But generic doesn’t inspire, and her voting record isn’t particularly noteworthy, since many of her colleagues have the same voting record. In fact, some of them have led on issues that Vukmir has only voted on.

Vukmir is not the ideal candidate for this Senate general election. She lacks the fresh biography of a newcomer to politics, the military and business credentials that give Nicholson instant credibility on foreign policy and economic issues, and the national support the GOP nominee will need to beat a well-funded incumbent.

She has repeatedly tried to skirt compliance with the state’s open records law, a transparency-in-government measure beloved by both right and left in the state. Her refusal to turn over records led to a lawsuit in 2013 that cost taxpayers thousands of dollars when Vukmir lost the case. Two years later, she cast a committee vote in favor of gutting the law. In the public outcry that followed, fellow lawmakers backtracked and admitted they were wrong: Vukmir did not.

Outsiders have a good track record in Wisconsin. Vukmir’s timing as a candidate for this Senate seat is less than desirable: being a 16-year politician isn’t part of the formula state Republicans have used to win federal races.

Nicholson’s timing, on the other hand, is impeccable. Donald Trump turned the state red in 2016 for the first time since the 1984 presidential election. Outgoing-House Speaker Paul Ryan endorsed as his replacement a newcomer who has never held elected office: Bryan Steil. State lawmakers even cleared themselves of the field when Steil announced his candidacy for Ryan’s district, WI-01. Sen. Ron Johnson, one of the Republican Party of Wisconsin’s greatest recent successes — was an outsider who ran against Democratic incumbent Sen. Russ Feingold’s status as an 18-year politician in 2010 and won against all odds, and then won his re-election bid, also against Feingold, in 2016. Mike Gallagher, another young Marine veteran, businessman and outsider, won a hotly-contested open congressional seat in Wisconsin in 2016.

At the state GOP convention this year, Gallagher was the only member of the state’s federal delegation to speak to the generally older party delegates about the need to recruit younger people into the party. His admonition — though ironic given the circumstances — is timely. State Republicans should embrace (and many have) young conservatives like Nicholson, who have the strength of conviction to walk away from the Democratic Party and are willing to back up their beliefs with action by running for office.

Wisconsin Republicans have a golden opportunity Tuesday to give Baldwin a run for her money — both literally and figuratively — this November. Nicholson clearly has the fundraising capacity, the outsider status, the veteran and business credentials, the enthusiasm and the conservative beliefs to beat Baldwin on Nov. 6. There’s a lot on the line right now. For goodness’ sake, Wisconsin, send in the Marine.