Grassroots conservatives and national Republicans face off once again in tonight's U.S. Senate Republican primary election in Wisconsin.

Marine Corps veteran Kevin Nicholson is supported by Tea Party groups like the Club for Growth, FreedomWorks, and the Tea Party Patriots Citizens Fund. He has also been endorsed by Sen. Ted Cruz, R-Texas. Nicholson is a former Democrat, once serving as the president of College Democrats of America. But after serving combat tours in Iraq and Afghanistan from 2007 to 2009, working in business as a management consultant, and having a family, he says he's become a conservative.

Running against him with the backing of Wisconsin GOP leaders and the endorsement of House Speaker Paul Ryan, R-Wisc., is state Sen. Leah Vukmir. She is campaigning as a consistent conservative and highlighting Nicholson's history as a Democrat.

The three other candidates running are Charles Barman, Griffin Jones, and George Lucia.

Wisconsin uses an open primary system, meaning voters do not need to register with a political party to vote in that party's primary elections. The polls will close at 9:00 p.m. ET.

Live results: