The U.S. Supreme Court began deliberating a case Monday that could dramatically curb the financial and political power of public-sector unions in Minnesota and more than 20 states around the country.

“It is hugely significant,” said David Larson, a professor of labor and employment law at the Mitchell Hamline School of Law. “It has to do with the viability of labor unions and their ability to finance themselves.”

A group of local union workers gathered Monday at Minnesota AFL-CIO headquarters to track the oral arguments on social media and discuss the possible implications on their own lives.

The case will decide whether unions have the power to require nonmembers to pay union dues. Under current law, a union is allowed to collect dues from all workers — not just members — when they negotiate wages, benefits and other workplace conditions.

A favorable ruling for the plaintiffs could cost unions millions of dollars in dues, money that labor groups have relied on to help mostly Democratic candidates and causes. Republicans have tried for decades to erode union influence in politics, and also to limit their negotiating power with businesses and governments.

“If unions lose resources, workers lose power,” said Jennifer Munt, spokeswoman for AFSCME Minnesota Council 5. “It’s really that simple.”

Supreme Court justices on Monday seemed critical of the idea that unions should have a guaranteed right to impose dues on nonmembers, leaving open the possibility that the practice would come to an end.

The plaintiffs, 10 California teachers, argue that even basic union tenets — such as contract negotiations — should be considered political activity because public-sector unions are negotiating with the government over pay, benefits and other workplace issues. Moreover, they argued that some positions by teachers unions, including support for layoffs and school assignments based on seniority, were at odds with their own views, making them effectively paying for the unions to argue against their own best interests.

A decision could have a huge impact in Minnesota, a state with a relatively strong labor movement that has played a role in nearly every corner of state and local politics. Minnesota is among 19 states that have higher union representation than the national rate of 11.1 percent, according to a 2014 report by the U.S. Bureau of Labor Statistics. In Minnesota, 360,000 workers are members of unions, for a rate of 14.2 percent. Counting nonmembers whose jobs are covered by unions, that number rises to 380,000, according to the 2014 report.

Minnesota law stipulates that employees who don’t elect to be in unions pay no more than 85 percent of the dues that unions charge full members.

Minnesota labor groups have held at least two events in recent weeks trying to highlight what they say is the positive role of union workers in the state. Union leaders said the court case represents broader attempts to reduce the power of public-sector unions, which ultimately will harm workers.

Education Minnesota, the statewide union representing 70,000 educators, collected nearly $28 million in union dues, according to 2014 tax records. AFSCME Local 5, which represents many state, county and municipal government employees, collected $21 million in dues that year.

Labor unions have used the money to fight for their workers and have become a powerful political force in the state.

Union contributions dominated donations to state political action committees, according to a Star Tribune analysis in 2013. State and national unions spent $17 million in Minnesota campaigns between 2007 and 2012. Over the same period, businesses directly donated about $3 million.

Munt and other union leaders say an adverse ruling could lead to some workers opting out of paying union fees but still retaining the benefits of union representation. The court “would effectively say the union has to provide its services free of charge to workers who don’t want to pay,” Munt said.

Keith Downey, chairman of the Minnesota Republican Party, said the case boils down to individual freedom. “If you think the union is doing a good job for you, and adding value, and speaking effectively for you, then you should be able to join the union,” he said. “If you don’t think that, you should be free to not join the union.”

The case brought to the Supreme Court this year was led by the Center for Individual Rights, a libertarian-leaning group backed by some prominent conservative organizations that have long sought to curb the power of public-sector unions.

One Minnesota conservative group, the Center of the American Experiment, says more people would be attracted to teaching if they didn’t have to pay union dues.

Nonmembers may be paying for political speech they don’t agree with, effectively snuffing out their own free speech. “You can’t leave the Constitution at the schoolhouse door,” said Kim Crockett, the center’s chief operating officer.

Some Minnesota educators disagreed, arguing that it’s only fair that nonmembers contribute toward covering the cost of negotiating contracts that benefit all of them.

“Everyone benefits from the organizing that the union does,” said Shannon Jax, a special-education teacher in St. Paul. She noted that teaching contracts cover nonpolitical aspects of work, including class sizes and safe work environments, an issue that has taken outsize prominence in St. Paul following recent assaults on teachers by some students.

Ron Hustvedt Jr., a middle school social studies teacher from Elk River, said an adverse ruling would be harmful to students and families.

“What teachers fight for is not more pay and benefits,” he said. “What we fight for are working conditions that allow us to better serve students.”