By Brendan O'Brien
MILWAUKEE (Reuters) - Wisconsin Democratic primary voters go to the polls Tuesday to pick a challenger to face Republican Scott Walker in a recall election next month, a contest that has captured national attention as a test of labor union strength and a gauge for how the battleground state may vote in November.
Leading in the public opinion polls is Tom Barrett, the Milwaukee mayor, who had a significant lead over Democratic candidates going into Tuesday's primary.
Barrett lost the Wisconsin governor's race to Walker by 5 percentage points in the Republican sweep of 2010. Since then, Wisconsin has been split by what Barrett calls a "civil war" over Walker's drive to curb union power in the state.
Walker infuriated Democrats and labor organizations weeks after taking office in 2011 by pushing a measure that curbed collective bargaining power of public sector unions through the Republican-led legislature.
The measure requires public sector union members to pay a portion of their pension and health care benefits and caps wage increases. It also requires most public unions to recertify each year and makes it voluntary to pay union dues.
The law set off massive protests at the capitol in Madison. In January, Walker opponents submitted more than 900,000 recall petition signatures to the state elections board, triggering a June 5 recall election of the governor.
"Democrats are so angry with Walker, this election is about the 'Not Walker' factor," said Timothy Dale, assistant political science professor at University of Wisconsin, Green Bay.
If he loses, Walker would become just the third U.S. governor to be removed from office by recall election.
Democrats surveyed by a Marquette Law School poll last week had Barrett at 38 percent, former Dane County Executive Kathleen Falk at 21 percent, Secretary of State Doug La Follette at 8 percent and state Senator Kathleen Vinehout at 6 percent. Nineteen percent of Democrats were undecided.
The same poll showed that if Barrett gets a rematch with Walker in the recall vote, they are in a virtual tie among registered and likely voters.
Almost everyone in Wisconsin has already decided what they think of Walker, leaving only 5 percent to 6 percent undecided.
The key to the outcome is likely to be which side does the best job of turning out its vote, analysts said.
The Walker attack on unions has not been the major issue of the campaign so far. Polls show Wisconsin voters, like those across the nation, are more focused on jobs.
Democrats have hammered Walker since a U.S. Bureau of Labor Statistics report showed Wisconsin lost 23,900 jobs, the most for any state, from March 2011 to March 2012.
Vast amounts of campaign contribution money are flooding into the state with conservatives around the country supporting Walker and unions backing the Democrats. Walker raised $13 million from January 17 through April 23, according to finance reports filed with Wisconsin - more than seven times the combined amount raised by Barrett and Falk.
The state's elections board expects voter turnout to be between 30 percent and 35 percent. This is high, compared with typical September partisan primaries in Wisconsin, which range from 9 percent to 25 percent turnout.
(Reporting by Brendan O'Brien; Editing by Mary Wisniewski and Lisa Shumaker)