Democrats are trying to stir some controversy against Republican U.S. Senate candidate Kevin Nicholson, a Delafield businessman and U.S. Marine veteran.

They're not going after Nicholson personally. Instead, they're aiming at one of Nicholson's big backers, GOP mega donor Richard Uihlein, co-owner of Pleasant Prairie-based Uline Corp.

Democrats claim that Uihlein is trying to "buy" the Senate seat for Nicholson.

They point to $15 million of donations in the past two years that Uihlein made to groups that now support Nicholson. Most of that cash, $10 million, went toward backing Republicans in the 2016 campaign.

So far in the current campaign cycle, the most prominent move by Uihlein is the $3.5 million he gave to the group Solutions for Wisconsin, which on Thursday dropped its first TV ad in support of Nicholson.

Uihlein also gave $1.5 million this year to Club for Growth Action, whose super PAC endorsed Nicholson.

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The Nicholson camp isn't buying the Democratic criticism.

Instead, it's taking the attack as a sign that Nicholson is making significant strides in the campaign to take on Democratic U.S. Sen. Tammy Baldwin in 2018. Nicholson and state Sen. Leah Vukmir of Brookfield are the only announced Republican candidates.

"Democrats are afraid of Kevin because they know that he is a conservative outsider and decorated Marine Corps combat veteran who will defeat Tammy Baldwin next November," said Michael Antonopolous, a Nicholson spokesman.

Richard Uihlein and his wife, Elizabeth, who live in Lake Forest, Ill., were big players in Wisconsin and nationally during the 2016 campaign.

The Uihleins provided major support to Republican U.S. Sen. Ron Johnson during his successful re-election over Democrat Russ Feingold.

Democrats have zeroed in on Uihlein's donations to groups that now support Nicholson — Solutions for Wisconsin, Club for Growth, Great America PAC, Restoration PAC, FreedomWorks for America and Americas PAC.

Two other Uihlein-supported groups also endorsed Nicholson, the Madison Project and John Bolton PAC.

Brad Bainum, a spokesman for the Democratic Party of Wisconsin, said in a statement: "Despite support from numerous out-of-state special interest groups, Kevin Nicholson can't find anyone from Wisconsin to back his candidacy. That's why his top supporter — a billionaire CEO from Illinois who can't even vote in next year's GOP primary — is throwing money at the problem, adding to the $15 million he's already invested in trying to buy Nicholson a Senate seat."

A source close to Uihlein refuted that accusation.

"While he obviously supports Kevin, he has not been involved in suggesting to other groups that they contribute to his campaign," the source said.

Chris Schrimpf, a spokesman for Solutions for Wisconsin PAC, also dismissed the Democratic claim.

"It sounds like Democrats are rightly scared about Kevin's campaign," Schrimpf said. "They know the same thing that we know; Kevin Nicholson will defeat Tammy Baldwin next year."