Thousands of protesters descended on Michigan's legislature Tuesday as Republican lawmakers passed union-curbing "right-to-work" bills in a state seen as the heart of the labor movement.

Once signed into law by Republican Governor Rick Snyder, the two measures will weaken unions by allowing public and private sector workers to get the same wages and benefits as union members even if they refuse to pay any dues.

Democratic lawmakers pleaded with their Republican colleagues not to pass the controversial bills, which they warned would unleash deep social and political strife.

"There will be blood. There will be repercussions," state representative Douglas Geiss told the House chamber.

Geiss reminded his colleagues of the violent clashes that accompanied the struggle to form unions in the 1930s and warned that people feel just as strongly about solidarity today.

"If 10 people walk in and say I'm not going to pay dues anymore, there's going to be fights," he said.

At least two people were arrested and troopers deployed pepper spray as the protest got rowdy in Lansing, state police said.

Currently, Michigan has 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.

The right-to-work law creates an incentive for people not to join the union -- in what is known as the "free rider" problem -- because it allows them to benefit from collective bargaining without paying for it.

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.

The business-backed measures had previously been restricted to states -- in the south and western 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 struggles to recover 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.

But while business may profit from weakening unions, the real motivation for lawmakers is political, said Roland Zullo, a labor relations expert at the University of Michigan.

"This whole right-to-work thing is retribution," Zullo told AFP. "It's really about the fact that unions in Michigan were very important actors in helping to elect Democrats this last election."

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 bills in an appearance at a Michigan auto plant 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 a cheering crowd of unionized workers.

"What they're really talking about is giving you the right to work for less money."

Boos and chants of "veto" poured into the chamber from the gallery after the House voted 58-51 to pass the first bill, which covers public workers. The second bill, which covers workers in the private sector, passed 58-52 after another impassioned debate.

Hundreds of union members and supporters crowded into the capitol dome, blowing whistles and chanting "the people are united" and "What's disgusting? Union busting!"

Thousands more shivered in the cold outside, television news footage showed.

"The right wing forces in Michigan are trying to take power away from working families," United Auto Workers union chief Bob King told reporters.

"They want working families to have less income, less security. This is about partisanship, not bringing the state together."

Republican state representative Lisa Lyons insisted that the law was about giving workers their constitutionally guaranteed right to freedom of association.

"We are witnessing history in the making," she told the House chamber. "This is the day that Michigan freed its workers."