MADISON, Wis. — For decades, states across the South, Great Plains and Rocky Mountains enacted policies that prevented organized labor from forcing all workers to pay union dues or fees. But the industrial Midwest resisted.Those days are gone. After a wave of Republican victories across the region in 2010, Indiana and then Michigan enacted so-called right-to-work laws that supporters said strengthened those states economically, but that labor leaders asserted left behind a trail of weakened unions.Now it is Wisconsin’s turn. On Monday, Gov. Scott Walker — who in 2011 succeeded in slashing collective bargaining rights for most public sector workers — signed a bill that makes his state the 25th to adopt the policy and has given new momentum to the business-led movement, its supporters say.