The Supreme Court is expected to rule on Monday, the last day of its term, in a landmark case that unions fear could deal a fatal blow to their movement.

The case, Harris v. Quinn, is about the constitutionality of “agency fees” charged by public sector unions to all workers in a unionized setting, even non-union members. These fees are essential to their operation.

The particulars of the case concern Medicaid-based home health care workers in Illinois. The battle dates back to a 2003 executive order by Gov. Rod Blagojevich (D) which paved the way for the Service Employees International Union to become the exclusive agent representing state home health workers. Then in 2009, Gov. Pat Quinn (D) issued another executive order classifying home care providers as state workers, and therefore eligible for exclusive union representation. In 2010, a group of home health workers, led by Pamela Harris, brought a class action lawsuit alleging that the collective bargaining agreement that required non-members to pay union fees violated their First Amendment rights.

Unions fear the implications extend far beyond the home health worker profession in Illinois. Agency fees in principle are important to public employee unions because they’re required by law to bargain for all workers in a unionized setting. If agency fees for non-members are ruled to be a violation of free speech, unions fear they would lose funding, become less effective at bargaining for benefits and, in turn, lose members.

A death spiral.

One labor official said such a result would bring about “the possible final destruction of the American labor movement.” The official added, “It would cause the death not only of public sector unions and what’s left of private sector unions, but also the Democratic Party,” suggesting that the demise of unions would make Democrats more reliant on Wall Street money.

Joel Rogers, a law professor at the University of Wisconsin, wrote in The Nation magazine that the challengers’ case in Harris goes for the “kill shot” against public employee unions.

Indeed, conservative and libertarian legal advocates see a golden opportunity for a victory against public sector unions. The Cato Institute, a libertarian think tank, wrote in a friend-of-the-court brief arguing, “There can be no question but that Illinois’s scheme to compel personal assistants’ association with, and subsidization of, labor unions flunks traditional First Amendment scrutiny.”

The makeup of the Supreme Court, which breathed life into this challenge in the 2012 case SEIU v. Knox, gives unions enough reason to worry. But a new twist on Thursday gave them even more reason to be concerned. Two major opinions on presidential recess appointments and abortion-buffer zones were written by Justice Stephen Breyer and Chief Justice John Roberts, respectively. That raises the odds that Justice Samuel Alito, who has a strong anti-union bent, will write the opinion in Harris, given that the justices tend to divide up opinions.

“Based upon who has written this term for each month, those of us who look at those statistics believe Alito could well have the opinion (although sometimes what happens is that someone is assigned a majority opinion and loses the majority on the way),” Rick Hasen, a law professor at UC-Irvine, said in an email. “Alito has shown himself very anti union in his remarks and earlier writings, especially when it comes to public sector unions and mandatory dues. This seems to be the thing he’s passionate about the way Chief Justice Roberts is about race issues.”

“Harris could be the sleeper case of the year,” Hasen said.