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.

"You know, these so-called right-to-work laws -- they don't have to do with economics. They have everything to do with politics," Obama told the cheering crowd of unionized workers.

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.

"You only have to look to Michigan, where workers were instrumental in reviving the auto industry, to see how unions have helped build not just a stronger middle class but a stronger America," Obama said.

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.

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.