They have the power to influence who runs for president and is elected to the White House, but teaching unions in the US are in danger of being dramatically weakened after a landmark lawsuit was filed in California earlier this month.
A group of 10 teachers and the Christian Educators Association International are bidding to overturn a law that requires all teachers to pay money to unions, whether they are members or not, for services including collective bargaining.
The complainants have filed their suit primarily against the California Teachers Association, an affiliate of the National Education Association, which is the country's largest teaching union. After California, they hope to take the case all the way to the US Supreme Court to set a national precedent.
About half of US states have labour laws that demand all teaching staff to pay a proportion of their wages to unions, which can be as much as $1,000 a year. But the plaintiffs argue that being forced to contribute to unions violates their First Amendment rights to free speech and association. They are also concerned that some of their money is used for political activity organised by unions.
If successful, the case would result in unions suffering serious financial losses, and could have significant implications for the wider political landscape in the US. Teaching unions are some of the largest political donors in the country, particularly to the Democratic Party: between 2003 and 2012, the California Teachers Association alone gave more than $150 million (#163;98 million) in political contributions.
Benjamin Sachs, a professor at Harvard Law School and an expert in labour law, said that the case could be hugely problematic for all public sector unions if it is upheld.
"What's clear from my perspective is if there were to be a holding from a court, particularly the US Supreme Court, that it is unconstitutional to require public sector employees - and in this case individual teachers - to pay union dues of any kind, that would be massively disruptive to public sector unions," he said.
The move would allow teachers to "free ride" on collective bargaining negotiations, he added, and unions could experience a significant reduction in funding as they would have to rely on voluntary payments alone.
In a statement, Rebecca Friedrichs, one of the teachers who lodged the complaint, said that individual members of the profession should not be forced to pay for unions' agendas.
"The union spends millions of teachers' hard-earned monies supporting causes and candidates that many of us oppose," she said. "It is shocking to me and many other teachers that union officials have the power by law to spend our wages to press for causes that many of us oppose on moral, fiscal or philosophical grounds."
The move is part of a wider push by the political Right to weaken public sector union power in the US. Although some states have enacted their own legislation to ban mandatory union contributions, many workers are still compelled to contribute.
The unions say that teachers already have the power to opt out of full membership, and fear that any decision in favour of the complainants would result in "free riding": teachers gaining from union negotiations without making any contribution.
Jacob F. Rukeyser, the California Teachers Association's staff counsel, said that the case could "significantly reshape US labour law" and overturn decades of legal precedent.
"We do not believe that is necessary or appropriate, and we hope that the case will not end up before the US Supreme Court," Mr Rukeyser said. "However, if the plaintiffs succeed in their stated goal, it would significantly alter labour relations in the US and hamper the ability of unions to represent their bargaining units.
"To be frank, this lawsuit seeks to undermine the effectiveness of all public sector labour unions."