Voters in Missouri will decide on Tuesday if the state adopts a “right-to-work” law hampering union organizing, a decisive moment in a years-long battle pitting labor and Democrats against an alliance of business groups and Republican politicians.

Republicans in Missouri’s state government in February 2017 approved a right-to-work law, preventing unions from signing contracts that force all workers to pay for union representation. The legislation undercuts organizing by allowing employees who are represented by the union to avoid paying fees, reducing the union’s resources.

Under Missouri law, new legislation can be put to a public referendum if about 100,000 state residents sign a petition to overturn it. Unions and their supporters last summer gathered 310,000 signatures to temporarily nullify the law until the vote. If a majority of voters rejects the law, the legislation will not go into effect.

Twenty-seven states already have right-to-work laws on the books, with Indiana, Michigan, Wisconsin, West Virginia and Kentucky all adopting them since 2012 largely because of Republican dominance of state legislatures in the Obama era and after. The laws are in keeping with conservatives' and business groups' support for less-regulated labor markets, and they also weaken unions’ political power, boosting Republicans by weakening a source of Democratic campaign money and organization.

The Missouri vote comes after defeats for unions at the national level and in other states, as Republicans have used control of the federal government and a majority of state legislatures to accelerate the reduction of organized labor’s influence to its weakest point in decades.

Overturning the law would give labor groups a victory after a Supreme Court setback in June. In the case of Janus v. American Federation of State, County, and Municipal Employees, the court ruled it was unconstitutional for public sector unions to require collective bargaining fees from workers.

Unions were already barred from using mandatory dues for campaign funding, but the ruling effectively allowed any worker to opt out of paying for any union activity. Union membership has declined in 20 of 26 states that have adopted right-to-work laws, according to Frank Manzo, policy director of the Illinois Economic Policy Institute.

Unions say employees who do not pay dues are “free riding,” enjoying the benefits of union-negotiated contracts without contributing to the costs of organizing. And they say workers would be hurt by the change, as weakened unions will struggle to negotiate higher wages and benefits.

But conservatives and business groups, including the ones financing the right-to-work push in Missouri, say right-to-work laws give workers greater freedom, relieving them of having to pay fees to a union they may not support or whose political organizing they find objectionable. Business groups in Missouri have also highlighted data they say shows faster economic growth in states that have adopted the laws and argued that workers can put more money in their pockets if not forced to spend it on the union.

“Workers should have the freedom to support their union,” said Jeremy Cady, state director for the Missouri branch of Americans for Prosperity, a conservative advocacy group. “Many unions provide great value to their workers. But some do not, and workers should have the ability to hold those unions accountable.”

State-level right-to-work laws extend that prohibition to private sector unions, which labor groups in the state say would make it harder to sustain their work.

“It would be a severe blow,” said Tom Madden, a spokesman with the St. Louis City Labor Club. “Missouri went for Trump, but a lot of the workers here are strongly against right to work, because they understand wages are going to go down, period.”

Union membership across the United States has already fallen markedly from the 1970s, with the percentage of American workers in a union falling from about one quarter in the 1970s to less than 11 percent in 2017, according to survey data.

The percentage of private sector workers in unions has driven most of that decline, falling from close to 25 percent in the 1970s to just above 6 percent in 2017, which some economists say explains tepid wage growth in the U.S. economy.

Liberals and union groups view Tuesday’s vote in Missouri as an opportunity to begin reversing the momentum that has weakened unions to their low point in several decades. About 226,000 workers in Missouri are members of unions, or about 9 percent of the workforce, according to federal statistics.

“We are seeing an attack on unions being sustained all over the place — the courts, private employers, the administration,” said Janelle Jones, an analyst at the Economic Policy Institute, a left-leaning think tank. “This vote could represent the pendulum swinging back to workers and away from corporate interests.”

The vote also may have immediate political consequences in a state with a closely contested Senate race, pitting vulnerable Democrat Sen. Claire McCaskill against Republican front-runner Josh Hawley, the state’s attorney general. Trump won the state by 19 points.

McCaskill has vocally opposed the right-to-work plan, while Hawley has supported it. Republicans in the state legislature voted to move the date of the right-to-work vote from November to August, in an apparent attempt to decouple it from the Senate race.

“At a time when working families are struggling because wages are stagnant and health care and education costs are soaring, nobody should be forced to pay union dues,” a spokeswoman for Hawley said in an email. “Sen. McCaskill has done nothing but exacerbate these problems.”

In a statement, McCaskill said that “there are few things more American than workers coming together to bargain for better wages, good benefits, and a safe workplace. . . . The Right-to-Work-for-Less proposal attacks workers and strips them of their basic rights.”

The stakes could extend beyond this election. Unions have traditionally constituted a core constituency in the Democratic Party, and weakening them has tended to help the Republican Party. Right-to-work laws have decreased Democratic presidential vote share by 3.5 percent, one study found.

In 2011, Ohio voters similarly roundly defeated a right-to-work proposal, dealing a blow to Gov. John R. Kasich (R), who had backed the measure.

Union organizers in Missouri say they are optimistic about a similar result, despite the state’s conservative orientation. Torre Brashers, a trucker from the small Missouri town of Clever, voted for Trump in 2016 and says he plans on voting against McCaskill in November.

But on Tuesday, Brashers is voting with the Teamsters union he says has helped deliver wages and health care for him and his family.

“I’ve been in the union for over 20 years, and I just think this would be bad for the working middle class,” Brashers said. “I’ve just seen how the union has benefited my middle-class family with wages and benefits. If this passes, it’ll have major effects to my family and other middle-class families.”