Wisconsin's Republican U.S. Senate candidates have signed a unity pledge proposed by U.S. Sen. Ron Johnson and the state GOP ahead of the 2018 election.

By agreeing to support the winner of the party's August primary, both candidates will be given access to a list of potential delegates who will vote at the party's state convention on who to endorse.

Although the delegates will be officially selected later this year, candidates may start seeking their support once given access to the list. 

State Sen. Leah Vukmir, R-Brookfield, and Delafield businessman Kevin Nicholson are seeking the Republican nomination to challenge Democratic Sen. Tammy Baldwin. 

Vukmir said she is "proud" to sign the pledge.

"As a lifelong Republican, I've always supported the party nominee and you can bet I'll be doing everything I can this November to make sure Wisconsin has a Senator who believes in the Wisconsin Way," Vukmir said in a statement. "Senator Johnson certainly understands that the grassroots are the core of Wisconsin's Republican Party, a focus I have built my candidacy on."

Nicholson spokesman Brandon Moody said Nicholson "of course will sign the pledge and he hopes that all Republicans will unite behind our nominee to defeat Baldwin."

His campaign tweeted an image of his signed form Wednesday morning.

Johnson will oversee the grassroots endorsement process at the state convention.

"The decisive ingredient in my two Senate race victories was our strong Wisconsin Republican Party grassroots structure in communities all across the state," Johnson said in a statement. "I know firsthand what happens when you are able to earn the trust of the grassroots party faithful at the Republican state convention — you can receive their crucial endorsement."

A political newcomer at the time, Johnson won the party's endorsement and a primary race in 2010 and went on to unseat Democratic Sen. Russ Feingold. He defeated Feingold in a 2016 rematch.

The senator said the pledge and the party's endorsement process are needed to prevent "national outsiders" from influencing in the race.

"Wisconsin needs a person of integrity supported by the good folks of our state to help advance conservative reform and take on Washington’s professional political class," Johnson said.

Democratic Party of Wisconsin spokesman Brad Bainum said the need for a unity pledge "shows how nasty their primary has become and it proves that — no matter who the nominee is — both Vukmir and Nicholson are willing to put their billionaire donors and special interests ahead of Wisconsin by driving up the cost of health care and working to cut Wisconsinites' Social Security and Medicare."

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