By James B. Kelleher

CHICAGO (Reuters) - Laws that weaken the power of organized labor could spread to more U.S. states in 2013 after supporters of the measures scored a major victory over unions in Michigan this week, and earlier in the year in Indiana, experts said.

The next battles over what advocates term "right-to-work" laws could be neighboring Midwest states of Wisconsin and Ohio, where Republican governors and legislatures have shown a willingness to take on the unions.

Missouri also could turn out to be the next flashpoint in efforts to end a "closed-shop" system that makes union membership a condition of employment.

Supporters say the new laws give workers the choice whether to join a union and pay dues. They also argue they encourage corporate investment. Critics say the laws undermine the basic tenet of union collective bargaining, suppress wages and strip workers of leverage to improve pay, benefits and conditions.

Michigan on Tuesday became the 24th of the 50 American states to enact such laws.

The outcome was a "catastrophe" for unions and a sign of their waning power, said Gary Chaison, industrial relations professor at Clark University's Graduate School of Management.

"Other states will be emboldened by the passage of the right-to-work law in Michigan," he said. "Before the next year is over, we will probably see a majority of states with right to work laws."

The outcome in Michigan struck organized labor particularly hard, because it is the birthplace of the United Auto Workers union and a symbol of union might. It followed a series of recent setbacks for the union movement from California to Wisconsin.

Before the Michigan eruption, Wisconsin was the epicenter of the debate over unions, but the issue was different. Republican Governor Scott Walker stripped public-sector unions of most of their bargaining power in 2011 but has not tried to enact right-to-work laws in the public or private sector there.

LINK TO BILLIONAIRES

"We will be pushing for a right-to-work bill here in Wisconsin," said Luke Hilgemann, the Wisconsin State Director for Americans for Prosperity. His group, linked to the billionaire Koch brothers, owners of an energy and trading conglomerate, was a significant force in getting the laws passed in Michigan.

After almost two years of acrimony with Wisconsin unions, Walker has avoided right-to-work in recent months and his office said the governor was focused on creating jobs, reforming government and improving infrastructure.

"Anything outside of that is a distraction," spokeswoman Jocelyn Webster said in an email earlier this week.

That statement is similar to words spoken by Michigan Republican Governor Rick Snyder just before he announced his support for right-to-work earlier this month.

Wisconsin State Assembly Democratic Leader Peter Barca, who has opposed such laws, said he is bracing for a new assault.

"We would be foolhardy... to not be skeptical and nervous," he said in an interview on Thursday. Continued...