LANSING, Mich. — Rick Snyder, the governor of Michigan, said he would sign so-called right-to-work bills that began moving through the Legislature on Friday, measures that would make the birthplace of the United Auto Workers the second state to curb labor this year.
The plan prompted demonstrations in the Lansing Capitol by hundreds of chanting union supporters, some sporting Santa Claus hats. Police used pepper spray and arrested eight members of the crowd before closing the building, said Shanon Banner, a State Police spokeswoman.
The measures in the Republican-dominated Legislature would end closed workplaces in which employees must pay union dues and fees. The bills would cover public and private employees, though police and firefighters would be exempt, the Republican governor said during a news briefing.
‘‘This is about workplace fairness and equality,’’ said Snyder, 54, who had said repeatedly that the issue was not on his agenda. ‘‘This is about the relationship between workers and their union. Workers should have a right to choose who they associate with.’’
The past year has been a time of historic tumult for organized labor, which in 2011 represented 11.8 percent of American workers, down from 20.1 percent in 1983, according to US Bureau of Labor Statistics.
Michigan, a seat of labor clout, would become the 24th right-to-work state if the Legislature approves the measure, following Indiana in February. Yet even in decline, union money and volunteers were crucial to the reelection of President Obama this year.
The Michigan House passed one of three bills as the Senate was poised to act on two others. The House vote was delayed when most Democratic members walked out in protest over the closing of the Capitol by the Michigan State Police because of the crush of demonstrators that packed the areas outside the two chambers. They returned about 15 minutes later when the doors were ordered opened.
Each chamber must approve the other’s bills, though the House-passed bill cannot be acted on in the Senate for five days.
‘‘This legislation is petty and vindictive politics at its most disgusting,’’ said Democrat Gretchen Whitmer of East Lansing, the Senate’s minority leader, on the chamber’s floor.
The three US automakers based in Michigan are neutral on the issue, according to e-mailed statements from Katie McBride of General Motors, Kevin Frazier of Chrysler Group, and Todd Nissen of Ford.
Voters are evenly split over whether Michigan should enact such a law, 47 percent in favor, 46 percent opposed, according to a poll by EPIC-MRA of Lansing. The survey has a margin of error of plus or minus 4 percentage points.
Lawmakers and Snyder ‘‘are walking on dangerous ground,’’ said Bernie Porn, president of the polling firm.
Snyder said only 17.5 percent of Michigan workers are union members, so the measure would affect relatively few.
‘‘It’s a shame this is happening in Michigan,’’ said Lee Graham, state training coordinator for the International Union of Operating Engineers. Graham, among protesters in front of Snyder’s office building, said the laws would make Michigan a less attractive place to work.
Richard Trumka, the AFL-CIO’s president, said in an e-mailed statement that the initiative is led by ‘‘a radical group of Republicans.’’
Similar measures were considered in 21 states this year, without passage. A right-to-work bill is also being discussed in Wisconsin, where Governor Scott Walker, a Republican, this year survived a recall after curbing collective bargaining for most public-employee unions. Snyder said it was time to take a stand. Indiana has siphoned off business and jobs, he said.
Snyder said he supports collective bargaining and that the proposed laws are not anti-union.
Whitmer on Thursday called the legislation ‘‘retribution’’ against unions that backed and lost a ballot issue in November to enshrine collective bargaining in the Michigan constitution.