Protesters hold wooden letters spelling the word “shame” in front of the Wisconsin State Capitol on March 10, 2011, in Madison, Wis. Thousands of demonstrators gathered to protest as the Wisconsin House voted to pass the state’s controversial budget bill, one day after Wisconsin Republican senators voted to curb collective bargaining rights for public union workers in a surprise vote with no Democrats present. (Justin Sullivan/Getty Images)

Update: This post has been updated after leadership elections in New Hampshire.

A month after voters handed Republicans control of an unprecedented number of state legislative chambers across the country, party leaders in key states are preparing a raft of proposals that would severely weaken the political power of labor unions.

The most immediate threat comes from so-called Right to Work legislation, which allow employees to opt out of joining a labor union. States without Right to Work laws allow unions to require employees to pay dues even if they decline to join because those unions negotiate businesses on behalf of all workers.

Republicans in at least five states — Wisconsin, New Mexico, New Hampshire, Ohio and Missouri — have introduced or plan to introduce versions of the law in legislative sessions that will begin in January. Legislators in Colorado, Kentucky, Montana and Pennsylvania are all likely to push similar laws, though union-friendly Democratic governors in each state will act as firewalls.

Wisconsin appears to be the most likely battleground between business interests and labor unions. State Rep. Chris Kapenga (R) said this week that he will introduce a Right to Work measure. Republican leaders including Gov. Scott Walker (R) have said such legislation isn’t a top priority, but Assembly Speaker Robin Vos (R) and Senate Majority Leader Scott Fitzgerald (R) have said they are open to bringing the issue up.

“The benefits to Wisconsin are pretty simple, and the statistics are very clear: If we pass legislation like right to work, we’re going to see an increase in the number of jobs available, and I fully expect to see an increase in incomes,” Kapenga said in an interview. “From a broad philosophical standpoint, this fits very well with what we’re doing.”

But there are risks involved, too: The last time Republicans advanced legislation curbing labor union powers, they were met with loud protests and recalls of everyone from Walker to state senators, both Republicans and Democrats. Walker survived the recall effort; several state senators did not.

The power of unions has waned in recent decades. The number of workers who are union members has declined in 43 states over the past decade. Just 11.3 percent of wage and salary workers were union members in 2013, according to the Labor Department.

Nonetheless, unions remain a potent political force. While the recall effort against Walker wasn’t successful, a 2011 Ohio law that would have limited the collective bargaining rights of public employees was overturned in a ballot referendum just months after it was passed. Future attacks on union power could risk similar blowback.

“Any introduction of right to work legislation would be a move in the wrong direction for Wisconsin and a serious disappointment at a time when Wisconsin needs legislators to focus on family-supporting jobs and reviving our sluggish economy,” Stephanie Bloomingdale, secretary-treasurer of the Wisconsin AFL-CIO, said in an e-mail. “These bills have proven time and time again to decrease wages and safety standards in all workplaces.”

New Mexico Republicans captured control of the state House this year for the first time in 60 years, and while Democrats control the state Senate, a coalition of moderates may join Republicans in passing a measure. The Democratic-controlled legislature passed right to work legislation in 1979 and 1981, though then-Gov. Bruce King (D) vetoed those laws both times.

Gov. Susana Martinez (R) supports right to work legislation, though she did not make it a cornerstone of her reelection campaign this year.

Republicans expanded their legislative majorities in Missouri, giving the party enough votes to override Gov. Jay Nixon’s (D) veto. Outgoing state House Speaker Tim Jones (R) made right to work legislation a top priority in 2014, though a measure fell four votes short of passage. Republicans picked up nine seats, giving the legislation new hope in the 2015 legislative session. Several draft bills have been filed, though one Republican state legislator said efforts to pair right to work legislation with other, more aggressive measures could complicate passage.

New Hampshire Republicans have said they will make a right to work bill one of their top priorities this year. But on Wednesday, former House Speaker Bill O’Brien (R), a major right to work proponent, lost his bid to return to office, throwing this year’s agenda in flux.

Twenty-four states have right to work legislation on the books. The debate tends to pop up in waves, said Jeanne Mejeur, an analyst at the National Conference of State Legislatures. Republicans began pushing the first versions in the 1940s and 1950s, when 17 states, mostly in the South and Midwest, passed bills. Michigan and Indiana were the two most recent states to pass right to work laws, in 2012.

Right to work “has been an area of renewed interest in the last few years,” Mejeur said.

Ohio proponents will use Michigan and Indiana as examples when they advance their right to work legislation next session. Two versions of the bill introduced this year will die with the end of the year, but Greg Lawson, a policy analyst at the conservative Buckeye Institute for Public Policy Solutions, said he expects right to work to return in 2015.

Right to work advocates learned their lesson from the 2011 battle over Senate Bill 5, Lawson said, and they plan to lay a more comprehensive foundation to avoid a public relations backlash once their measure is introduced.

“It’s going to take a lot of tilling of the soil,” Lawson said.

Ohio Gov. John Kasich (R) is one patch of soil that will need some attention. Kasich told the Cincinnati Enquirer’s editorial board this year that he hadn’t heard any complaints about right to work laws from CEOs looking to open facilities in Ohio. Still, he did not say definitively whether he would support or oppose any legislation that might land on his desk.

Most state versions of right to work laws look similar, said Greg Mourad, vice president of the National Right to Work Committee, even those passed more than half a century ago. The decision to allow right to work laws is given to the states under the National Labor Relations Act, passed in 1935.

Business groups like the Chamber of Commerce and conservative groups like the American Legislative Exchange Council favor right to work laws, which they say give workers more freedom and enhance economic development. Unions, which stand to lose huge amounts from their annual operating budgets if workers opt out, say those laws give businesses an opportunity to exploit workers by paying them less.

Union officials say they will aggressively push back against any measures that come up in legislatures across the country. Josh Goldstein, a spokesman for the AFL-CIO, said the union would educate and mobilize 2.5 million members in key states. The union also plans a national summit on raising wages, with regional summits in targeted states and cities.

But backers say recent economic growth in states that have right to work laws on the books bolster their arguments.

“The more states that get right to work, the more stark the numbers are in terms of economic benefits,” Mourad said. “Getting the roll call vote and putting all the politicians on the record is a short-term success, if not the total victory. Because once they’re on the record, people can hold them accountable.”