Labor has fought many battles this decade, but the fight in the highest court could be the biggest of them all.

Last week, the Freedom Foundation submitted two amicus curiae briefs supporting the petitioner in Janus v. AFSCME, the case in which a lone state employee (Mark Janus) is challenging the notion of paying union dues for public-sector unions. Janus, the lead plaintiff, and two other Illinois state employees believe that mandatory union dues violate their First Amendment rights.

President Donald Trump’s administration filed an amicus brief supporting the petitioner as well.

“Recent research shows ‘agency fee’ provisions undermine labor peace rather than promote it,” states one of the Freedom Foundation briefs. “This constitutes the death knell for the constitutionality of agency-fee provisions—which this court recently called a ‘jurisprudential anomaly.’”

The Freedom Foundation is a pro-free enterprise nonprofit that wants to “reverse the stranglehold public-sector unions have on our government.”

Under current law, every union-represented public sector worker has the choice to not join a union but is still required to pay dues on behalf of the union that negotiates salaries and benefits for all.

In 2016, Friedrichs v. California Teachers Association ended in a 4-4 split decision with the plaintiff arguing that forcing workers to pay union dues—and fund activities for an organization they don’t share values with—violates their First Amendment rights.

Janus’ case is bankrolled by the National Right to Work Foundation and the Liberty Justice Center (the litigation wing of the Illinois Policy Institute). Both are pro-business outlets.

AFL-CIO President Richard Trumka said that Trump boasts about supporting the working class but behind closed doors is threatening their livelihoods.

“For more than 40 years, through Republican and Democratic administrations, the law has recognized that unions and employers have the freedom to negotiate agreements under which everybody contributes his or her fair share,” said Trumka in a statement. “But now the Trump administration is urging the Supreme Court to reverse this precedent and undermine working people and our unions. This is a shameful political payback to reward those who seek to do working people harm.”

Trumka concluded, “Arguing against our freedoms at work is not what working people expect of our government.”

In 1977’s Abood v. Detroit Board of Education case, the Supreme Court ruled that workers were allowed to opt out of full union membership but still had to pay union dues. Labor leaders argued that because all workers are paid union wages regardless of membership, even unaffiliated workers should pay contract negotiating costs.

Lee Saunders, president of AFSCME, said that this case is just another line of attack in the war against unions.

“For the Solicitor General’s Office to reverse positions under the Trump administration—when they have argued in support of the Abood precedent twice in the past four years—underscores the fact that this case is a purely political attack on the freedoms of working people,” said Saunders in a statement. “The true motivation behind this case is to satisfy the agenda of wealthy and powerful political donors who have spent tens of millions of dollars to, as they have said in their own words, ‘defund and defang’ the American labor movement.”

In February, congressional Republicans introduced a bill that would make union dues voluntary for all American workers.