* Opponents say draft law aimed at undermining unions
* Percentage of union workers is fifth highest in nation
* Obama opposes "right-to-work" measures
By Andrea King Collier
LANSING, Mich., Dec 6 (Reuters) - Michigan's Republican-controlled Legislature passed separate bills on Thursday aimed at making the home of the U.S. auto industry the 24th "right-to-work" state banning mandatory union dues, igniting raucous demonstrations that led to eight arrests.
The Michigan House of Representatives voted 58-52 to approve a measure that would make payment of union dues voluntary in the private sector, after Democrats walked out in protest at the public being kept out of the Capitol.
A few hours later, the state Senate passed two "right-to-work" bills for private- and public-sector workers on 22-16 and 22-4 votes. Each measure must be sent for consideration to the other chamber before receiving final legislative approval.
Republican Governor Rick Snyder has pledged to sign the bills when they are sent to him. Snyder, who said last year that "right-to-work" legislation would be divisive for the state, said this week he now supported it.
The House has adjourned until Tuesday, the next date when it could take up the bills sent to it from the Senate and the Senate could take up the House bill under Michigan rules.
Thousands of union workers converged on the state capital, Lansing, to protest the sudden drive f or the law, and officials closed the Capitol building's doors for hours, citing safety concerns.
Eight people were arrested when they tried to rush past state troopers outside Senate chambers during the demonstrations and two people were hit with mace in the commotion, State Police Inspector Gene Adamczyk said.
The protests recalled the bitter two-year fight in Wisconsin, where Republicans voted to curb the powers of public-sector unions.
The 2011 Wisconsin law sparked massive protests and an unsuccessful effort to recall Republican Governor Scott Walker.
Approval of such legislation in the union stronghold of Michigan would be a major blow to organized labor. Michigan is where the headquarters of General Motors, Ford Motor Co and Chrysler are located, and is the home of the United Auto Workers union.
"It's another attack on the middle class," UAW President Bob King said in an interview. "The wealthy are never satisfied."
King said that if the law were passed, the union would try to overturn it at the ballot box through a referendum.
The bills include appropriations and may be ineligible for challenge by ballot initiatives.
REPUBLICANS LOOK FOR PASSAGE BY YEAR-END
Republicans intend to pass separate measures applying to public-and private-sector unions. They are rushing to finish by the end of the year because Democrats gained five seats in the state House in the November election, narrowing the Republican majority in the new Legislature convening in January.
Michigan would be the second state in the industrial U.S. heartland to adopt such a law, after Indiana earlier in 2012, and the 24th in the nation, although most are in the South.
"Right-to-work" laws typically allow workers to opt out of paying union dues and bar requirements that an employee must join a union to work in a certain shop.
Supporters say the laws help attract or keep businesses, while opponents say they suppress workers' wages and benefits and are aimed at undermining the financial stability of unions.
A White House spokesman said President Barack Obama had long opposed "right-to-work" laws.
"The president believes our economy is stronger when workers get good wages and good benefits, and he opposes attempts to roll back their rights," spokesman Matt Lehrich said.
All three major U.S. automakers said they were "neutral" on the proposed law, although the Michigan Chamber of Commerce said it supported the measure.
Republicans were emboldened to seek the measure after Michigan voters in November rejected a measure that would have enshrined a right to collective bargaining in the state constitution.
Michigan had the fifth highest percentage of workers in the country who are union members in 2011, according to the U.S. Bureau of Labor Statistics.
Protester Kathleen Tronjo, a teachers' union member from Port Huron, Michigan, said the schools and children would suffer under "right-to-work" laws.
"This is just bad policy," Tronjo said. "They just keep taking more and more away from teachers."
At a news conference on Thursday, Snyder said, "Quite often people call it right-to-work, but I think it is a much better description to say that this is about fairness in the workplace and equality in the workplace."
Unlike some other Republican governors who have championed curbs on unions, Snyder acknowledged he was a reluctant supporter of the measure.
"We have come to the point over the last few weeks and the last month or two where that issue was on the table whether I wanted it to be there or not," he said.