President Barack Obama on Monday threw his support behind labor unions opposed to a Republican-led drive for "right-to-work" laws in Michigan, saying efforts to pass such measures were not about economics but about politics.

Obama used a visit to an auto plant in the cradle of the American labor movement to weigh in on the controversial push in the state legislature to impose new restrictions on unions, part of the Democratic president's political base that helped him win re-election last month.

"What we shouldn't be doing is trying to take away your rights to bargain for better wages and working conditions," he said to loud cheers from workers at the Daimler Detroit Diesel plant in Redford, Michigan. "We shouldn't be doing that."

"These so-called right-to-work laws, they don't have to do with economics, they have everything to do with politics," Obama said. "What they're really talking about is giving you the right to work for less money."

Union members and others opposed to Michigan becoming a right-to-work state plan major protests in the state capital Lansing this week. Organizers expect thousands at a rally on Tuesday when the state legislature reconvenes.

With Republicans in control of the legislature and the Republican governor committed to sign the laws, Michigan could become the 24th right-to-work state by the middle of the week, dealing a stunning blow to the power of organized labor in the United States.

Michigan Republicans surprised labor unions on Thursday by pushing through the legislature in a day a proposal making union membership and dues voluntary in the private sector. The state Senate also voted to apply that to the public sector, except for police and fire unions.

Unions may not be able to stop Michigan Republicans from making the laws final on Tuesday, political analysts said. Governor Rick Snyder, who once opposed right-to-work as a divisive issue, has said he will sign them into law.