"There will be blood," Representative Douglas Geiss, a Democrat from Taylor, said during debate in the House of Representatives.
Outside the Capitol, supporters and opponents clashed, with protesters tearing down a tent set up by Americans for Prosperity, an organization supported by billionaires Charles and David Koch. They overturned tables and stamped on signs with slogans such as "Stop feeding the union pigs."
Police on foot and horseback charged through the crowd, pushing them back with batons.
Bill Bagwell, a 55-year-old UAW member from Westland, said the measure would create friction at his General Motors Co. plant in Ypsilanti. Members who pay union dues would detest those who enjoy the benefits of the contract but don't contribute, he said.
"It'll create civil war," Bagwell said.
Michigan's push for the laws, which exclude police and firefighters, began last week, when Snyder ended more than a year of neutrality and declared he would sign the legislation. Hours later, it won initial approval in the House of Representatives and Senate with no hearings.
Democrats failed to strip from the bills a $1 million appropriation to administer the measures. The inclusion of the money will shield the laws from a referendum to repeal them.
Supporters say right-to-work laws give workers the option of withholding support from unions they view as ineffective or politically objectionable.
"Unions will be more responsive and more jobs will come to Michigan," Snyder said at the briefing. "I try to do what's best for the citizens of Michigan."
The dues issue came to a head after unions spent $23 million in an unsuccessful campaign to enshrine collective- bargaining rights in the state constitution with a ballot measure in November. Snyder had asked union leaders not to seek the constitutional amendment, and he campaigned against it, saying it would undo efforts to rein in employee costs.
President Barack Obama, who was the beneficiary of union contributions in his re-election campaign this year, said yesterday in a visit to Michigan that "we don't want a race to the bottom."
"What they're really talking about is giving you the right to work for less money," Obama told workers at the Detroit Diesel plant.
The events in Michigan, with a history of combative organizing and powerful ties to the UAW and International Brotherhood of Teamsters, are watched by unions as a possible harbinger of similar campaigns in other states. Opponents say the laws are an attempt to strip unions of money used not only to bargain with management but to support political campaigns.
About 17 percent of Michigan's workforce belongs to unions, according to the U.S. Department of Labor. In the early 1960s, about 40 percent did.