When Wisconsin airwaves begin drowning in political ads this year, voters in the Badger State will have an Illinoisan to thank.

The U.S. Senate race in Wisconsin has drawn by far the most outside spending of any 2018 midterm election contest. And the vast majority of it has come from a single donor: Lake Forest, Illinois, businessman Richard Uihlein. The founder and CEO of shipping product giant Uline has vaulted himself to the upper echelon of Republican megadonors. And the Wisconsin senate race is his number one target.

Uihlein has poured millions directly into the contest by way of a pair of super PACs that he funds almost single handedly. But those donations only scratch the surface of the money he’s injected into the race. Uihlein has also provided major cash infusions of late to more established conservative groups that have endorsed his preferred Wisconsin senate candidate, Delafield businessman and Marine Corps veteran Kevin Nicholson. Meanwhile, Uihlein’s family foundation has written large checks to nonprofit groups that, though officially nonpartisan, have quietly boosted Nicholson’s candidacy with ostensibly apolitical activities targeting a Republican primary rival and Nicholson’s would-be general election opponent.

A descendant of noted the Milwaukee brewers beer brand Schlitz, Uihlein, who did not respond to a request for comment on this story, lives north of Chicago, and his company is headquartered just across the state line, in Pleasant Prairie, Wisconsin. He’s jumped into previous high profile races in Wisconsin, including providing major financial backing for Gov. Scott Walker’s gubernatorial campaigns. In May, Uihlein cut a $250,000 check to the state Republican Party.

But Uihlein has really flexed his financial muscle in past federal elections. He and his wife were the ninth largest individual political donors in the 2016 cycle, shelling out nearly $24 million to elect President Donald Trump and congressional Republicans, according to the Center for Responsive Politics. But in 2018 cycle, Uihlein has already claimed the top spot among the nation’s most deep-pocketed political donors. He’s even outpacing Democratic megadonor Tom Steyer, who has held the top spot since 2014.

The bulk of Uihlein’s financial heft has gone to two super PACs backing Nicholson’s primary bid against State Senator Leah Vukmir, and taking potshots at incumbent Democratic Sen. Tammy Baldwin. Democrats are expected to hold the seat, but Uihlein has nonetheless plowed $3.5 million into the group Solutions for Wisconsin—he is the group’s only publicly disclosed contributor—which has run ads backing Nicholson’s candidacy. Solutions has passed along $750,000 to another super PAC, Restoration PAC, which has spent the bulk of its funds attacking Baldwin, though the group has also gone after vulnerable West Virginia Democrat Sen. Joe Manchin.

Other Uihlein-backed groups have also jumped in on Nicholson’s behalf.

The John Bolton Super PAC—the former UN Ambassador’s political group, to which Uihlein has donated $450,000—announced a $1 million ad buy for Nicholson last month. Great America PAC, a pro-Trump political group to which Uihlein has donated $100,000, endorsed Nicholson in October. Uihlien has given more than $400,000 to the Madison Project, which backs insurgent conservative Senate candidates. That group officially endorsed Nicholson in August.

Also in August, Nicholson picked up a key endorsement from the conservative Club for Growth, to which Uihlein has donated more than $5.7 million since 2015. The Club has drawn fire from other conservative movement leaders, such as Grover Norquist’s Americans for Tax Reform, for its broadsides against Vukmir.

Like many GOP Senate primaries around the country, the contest between Vukmir and Nicholson has involved a degree of jostling over who can claim the mantle of Trump Republican. And as a result, like similar races, it went through some tumult in the wake of the high-profile split between the president and his former chief strategist, Steve Bannon, until recently a de facto leader of the Trumpian brand of nationalist politics that many Republican candidates hope to capture.

Bannon’s political operation got behind Nicholson’s candidacy early, and when the recently ousted Breitbart News chairman’s allegations of “treason” against members of the president’s political team became public, Nicholson declined to distance Bannon from his campaign.

Vukmir’s campaign quickly the “vicious attacks by Steve Bannon against the President and his family” and called on Nicholson to “disavow his endorsement.” Nicholson declined to do so. “Leah spent a great deal of time and energy seeking Steve Bannon’s endorsement and was unsuccessful. It’s easy to see why she is frustrated,” his campaign said.

Bannon’s involvement in the race may have been deeper than just a sought-after endorsement. According to a source close to Bannon, he had been in talks with Uihlein about coordinating the latter’s political spending. With GOP megadonors Robert and Rebekah Mercer publicly separating from Bannon, Uihlein could provide an alternate means of financing for Bannon’s political projects. But it’s not clear if any relationship came to fruition, or if the controversy that resulted in Bannon’s resignation from Breitbart last week ended discussions between the two.

Bannon is expected to announce a new “dark money” nonprofit group in the coming weeks, and the organization’s sources of financing are not yet known. Uihlein, for his part, funds a host of such conservative nonprofit groups that, while officially barred from taking sides in political contests, have boosted Nicholson’s candidacy behind the scenes.

In April, a conservative group called American Majority, which has received $250,000 from Uihlein’s family foundation since 2015, helped prevent another potential Republican candidate in the Wisconsin primary. The group circulated an opposition research memo highlighting a series of tweets sent by Nicole Schneider, a trucking heiress who was then considering a senate bid. The tweets included messages critical of prominent Republicans, including Trump, and praising Democrats such as Sen. Elizabeth Warren (D-MA). Schneider opted against a senate run.

Uihlein’s foundation has given another $250,000 since 2015 to the Americans for Prosperity Foundation, the 501(c)(3) “educational” arm of libertarian billionaires Charles and David Koch’s political activist organization. In October, AFP’s Wisconsin arm announced a “seven figure” ad campaign targeting Baldwin.

It’s common on both sides of the political aisle for high-dollar donors to fund nonprofits that can conduct non-electioneering activities in the de facto service of a political candidate, even as they shield the identities of their donors. It’s also a strategy that has worked out in the past for Uihlein, who funds a number of conservative nonprofits in Illinois that have backed candidates that Uihlein officially supports in an overtly political capacity.

Outside spenders in the Wisconsin senate race have already dropped ten times the sum of the next most expensive midterm contest. But even that total—more than $6.5 million, according to CRP—doesn’t tell the full story. Activities by dark money groups such as AFP and American Majority swell expenditures in the race, though those activities can be hard to trace.

Wisconsin’s August 14 senate primaries are relatively late in the contest, meaning expenditures in the race are likely to swell. And with the GOP senate majority on the line in a state where Trump scored an upset victory in 2016, the contest is sure to continue drawing large checks throughout the year.