Missouri voters on Tuesday rejected a “right to work” law that would have hampered union organizing in the state, the Associated Press has projected.

The result deals a setback to state Republicans who have long sought to make Missouri a right-to-work state, while handing a victory to labor groups whose power has been diluted by the Supreme Court and GOP-dominated state legislatures.

With 95 percent of precincts reporting, the AP said 67.3 percent of voters opposed the measure, while 32.7 percent supported it.

State Republicans in February 2017 approved a right-to-work bill, preventing unions from signing contracts that require all workers to pay for union representation, effectively allowing employees to opt out of paying dues.

But 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. Tuesday’s vote means it will not take effect.

Organized labor considers the victory in Missouri just one sign that unions and their allies in the Democratic Party can stage a comeback after a decades-long decline and repeated defeats, including recently at the Supreme Court.

In June, the Supreme Court ruled in Janus v. American Federation of State, County and Municipal Employees that it was unconstitutional for public-sector unions to require collective-bargaining fees from workers.

Missouri is not the first state President Trump won by a large margin to show signs of a resurgent labor movement. Teachers frustrated with low pay and reduced benefits have held protests with thousands of people this year in Arizona, Oklahoma and West Virginia, among other traditionally conservative states, and dozens of teachers tied to their unions are now running for state offices.

Meanwhile, Democrats have a chance of winning control of Congress in the November midterm election, and that could put some of labor’s top national priorities back on the agenda, including minimum-wage increases and paid sick leave. Labor organizers are also pondering how to undo right-to-work laws in some Midwestern states, although they first need Democrats to win back those statehouses.

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

“It shows just how out of touch the Republican legislature is with their constituents,” said Richard L. Trumka, president of the AFL-CIO. “The workers in that state don’t want anything to do with right to work.”

During Barack Obama’s presidency, Republican-dominated state legislatures enacted several curbs on unions at the state level, with Indiana, Kentucky, Michigan, Wisconsin and West Virginia adopting right-to-work legislation since 2012. (Twenty-seven states have right-to-work laws on the books.) Union membership has declined in 20 of 26 states that have adopted right-to-work laws, said Frank Manzo, policy director of the Illinois Economic Policy Institute.

Conservatives and business groups, including the ones financing the 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 also have 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 they are not forced to spend it on the union.

“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 Josh Hawley, the Republican nominee for the state’s Senate race, said in an email.

Unions have argued that employees who do not pay dues are “free riding,” receiving 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.

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