MILWAUKEE (AP) -- Polls are open in Wisconsin for a primary election that will determine who will take on the first-term governor in a historic recall election next month.

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Milwaukee Mayor Tom Barrett and former Dane County Executive Kathleen Falk are two of four Democrats on Tuesday's ballot who are looking to challenge Walker in the June 5 general election that has become a nationally watched battle over union rights. Since his successful push to effectively end collective bargaining rights for most state workers, Walker has emerged as a national conservative hero.

Other Democrats on the ballot are Secretary of State Doug La Follette and state Sen. Kathleen Vinehout. Gladys Huber is a Republican running as a Democrat.

Walker faces token opposition in the GOP primary from Arthur Kohl-Riggs, a Walker opponent running as a Republican.