MADISON, Wisconsin (Reuters) - Tens of thousands of demonstrators were expected to take to the streets of the Wisconsin state Capitol later on Saturday as protests against a Republican plan to curb the power of public sector unions entered their third weekend.

Labor groups also planned for large demonstrations in every state capital in the nation on Saturday to show solidarity with Wisconsin in fighting the proposal they see as trying to break the union movement.

Organizers and law enforcement were expecting another huge turnout in Madison, rivaling the estimated crowd of 50,000 last Saturday which was the largest protest since the demonstrations began.

But demonstrators will have to brave frigid temperatures in Madison, where the high is forecast to reach only 19 degrees Fahrenheit on Saturday.

So far cold temperatures have not deterred union members and their supporters, especially on the weekends. But this Saturday's protests take place in a different context, after the controversial union proposal passed the state Assembly on Friday, testing the resolve of a movement that has so far been remarkably peaceful.

Backers of the bill, including the state's new Republican Governor Scott Walker, won a major victory early Friday with state Assembly approval of the union bill. The bill has now moved to the state Senate where it is stalled because the chamber's 14 Democrats stayed away from the Capitol to deny Republicans a quorum to vote.

The Wisconsin changes sought by Walker would make state workers contribute more to health insurance and pensions, end government collection of union dues, let workers opt out of unions and require unions to hold recertification votes every year. Collective bargaining would be allowed only on wage increases up to the rate of inflation.

Early on Saturday morning, the scene around the Capitol building was quiet, with an overnight dusting of snow.

Capitol Police in Madison plan to end the overnight campout by protesters inside the Capitol building after Saturday night.

(Reporting by James B. Kelleher; Editing by Greg McCune)

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