Labor unions and Democrats slammed the White House Thursday for weighing in against public-sector unions in a major Supreme Court case, saying the Trump administration's opposition to mandatory union dues threatens the unions' ability to represent government workers.

"While President Trump boasts his support for working families, his administration is advocating a position in the United States Supreme Court that disregards decades of settled law and threatens our livelihoods. Yet again, his actions are failing his rhetoric and making clear that he has no intention of following through on his commitments to working people," AFL-CIO President Richard Trumka said.

In a friend of the court brief for the case Janus v. American Federation of State, County and Municipal Employees filed late Wednesday, the White House argued that the mandatory dues violated union members' First Amendment rights and should be abolished.

Rep. Mark Pocan, D-Wis., called the administration's position "stunning" and proof that he is governing for "the wealthy donor class" rather than voters. "With this brief, it is clear that President Trump has no plans to raise wages, create jobs, or grow and strengthen the middle class. Rather, President Trump is engaging in a coordinated effort with Republicans in Washington to dismantle unions, rig the economy against American workers, and give handouts to the wealthy and corporations. American workers deserve better," Pocan said.

Neither statement delved into the facts of the case, which involve if Mark Janus, an Illinois state government employee, can be forced to regularly pay a "security fee" to his workplace's union as a condition of employment. In the brief, the administration argued that because the unions negotiate with governments over issues involving budgets and size of government, they by definition involve political viewpoints and therefore workers can be forced into subsidizing views they disagree with.

"To compel a public employee to subsidize his union’s bargaining position on these questions is to force him to support private political and ideological viewpoints with which he may strongly disagree," the brief stated. "Public employees’ strong interest in freedom of speech on such matters outweighs the government’s interest in authorizing public-sector unions to collect agency fees that they do not need to fulfill their responsibilities."

Security fees are a common provision in public-sector union contracts. Unions such as AFSCME count on those fees. An internal survey the union did in 2015 found that only a third of their members would voluntarily pay dues no matter what, half of its membership couldn't be counted upon to do that. A minority of 15 percent would be certain to opt out of paying dues.