Walker is expected to easily win the GOP nomination, while the outcome of the Democratic primary is less certain. Milwaukee mayor Tom Barrett, the party's 2010 gubernatorial nominee, faces a strong challenge from former county executive Kathleen Falk. Additional Democrats are running for the nomination.
The winner will challenge Walker in a pivotal June 5 election that has garnered involvement from conservatives such as New Jersey Gov. Chris Christie, tea party members, and well as labor activists across the country.
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