Wisconsin Governor Scott Walker, a Republican presidential hopeful, signed a bill into law on Monday that stops private sector workers from being required to join a union or pay dues as a condition of employment.

The law takes effect immediately, making Wisconsin the 25th state to approve a so-called right-to-work law and marking the latest victory for Republicans targeting labor unions, following adoption of similar laws in Michigan and Indiana in 2012.

"It sends a powerful message across the country and around the world," Walker said, adding that what he called freedom to work is one of the three or four things that people ask about when he is on trade and investment tours for Wisconsin.

Walker became a hero to conservatives when he pushed for a law to limit the collective bargaining rights of public-sector employees shortly after taking office in 2011. His stature grew when he survived a union-backed recall election in 2012.

"This is one more tool that will help grow good paying, family supporting jobs here in the state of Wisconsin, particularly in the areas where you need them in manufacturing," Walker said.

Wisconsin's Republican-led legislature approved the bill in an extraordinary session, with representatives voting on Friday along party lines for the measure just two weeks after the Senate majority leader said the bill would be introduced.

Supporters said the law would attract businesses and jobs, while opponents said it was a thinly disguised assault on organized labor that will drive down wages and leave workers vulnerable.

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Thousands of workers demonstrated at the capitol in Madison as lawmakers debated the bill, but crowds were far thinner than four years ago, when tens of thousands of people protested the push for changes to public sector collective bargaining rights.

About 8 percent of private-sector workers in Wisconsin are union members, down from about 22 percent three decades ago, according to the Unionstats.com website that tracks membership.