A decades-long battle to cripple unions in the United States has shifted to the heart of the labor movement as Michigan’s Republican governor prepares to sign “right-to-work” legislation.

Unions are a key source of financial and grassroots get-out-the-vote support for President Barack Obama’s Democrats, and he was quick to slam the controversial bill in an appearance at an auto plant in the state on Monday.

He added that Republicans are “trying to take away your rights to bargain for better wages and working conditions,” in a move that undermines the American dream.

The Michigan measure would weaken unions by allowing workers who get the same wages and benefits as union members to decline to pay any dues.

Currently, the state operates a “closed shop” policy that requires workers who profit from collective bargaining to pay fees but does not make it mandatory for them to become union members. This creates an incentive for people not to join the union in what is known as the ‘free rider’ problem.

Michigan Governor Rick Snyder insists the law is necessary “to maintain our competitive edge” and attract new jobs, especially after neighboring Indiana became the 23rd state to enact right-to-work legislation earlier this year.

He is expected to sign it as early as Tuesday and unions are planning a march on the state capitol in Lansing in an attempt to change his mind.

The business-backed “right-to-work” laws had previously been restricted to states — in the south, west and center of the country — that already had a weak union presence.

The expansion to the industrial heartland states of Indiana and Michigan — birthplace of the United Auto Workers union — represents a major shift.

It comes as the Rust Belt is still recovering from major battles over efforts to radically restrict the collective bargaining rights of public sector workers in Wisconsin and Ohio after Republicans swept to power in 2010.

While unions and their supporters managed to overturn Ohio’s law in a referendum, the Wisconsin restrictions remain in place after failed efforts to oust the state’s Republican governor and regain Democratic control of the legislature through recalls.

Michigan’s congressional Democrats warned of “endless strife” if Snyder does not veto the bill or put the issue in front of voters in a referendum.

Snyder and his fellow Republicans will likely pay a “high political cost” for signing a law he had long said he was too divisive to table, said Harley Shaiken, a professor at the University of California, Berkeley who specializes in labor issues.

The backlash against attacks on union rights in Wisconsin and Ohio galvanized the labor movement and helped Obama win the November election, he said.

Unions remain a powerful political and economic force despite seeing membership fall from 20.1 percent of US workers in 1983 to 11.8 percent in 2011.

There are number of reasons why membership has dropped so precipitously, said Roland Zullo, a labor relations expert at the University of Michigan.

Globalization and the decline of manufacturing eliminated millions of union jobs, he noted. While court challenges overturned or weakened laws drafted to protect unions, Republican legislators enacted new measures to restrict unions further. And union leaders also made serious mistakes in responding to the changing economic, political and demographic realities.

But while business may profit from weakening unions, the real motivation for lawmakers is political.