By Brendan O’Brien, Chicago Tribune

Wisconsin Democratic primary voters went to the polls on Tuesday to pick a challenger to face Republican Scott Walker in a recall election next month, a closely watched test of labor union strength and barometer of political pressures in a battleground state ahead of November elections.

Leading in pre-primary public opinion polls was Tom Barrett, the Milwaukee mayor, who had a significant lead over a handful of other 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.

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