The U.S. Supreme Court poked a hole in the sails of public sector labor unions Monday in a ruling foes of forced unionization see as foundational for a later, larger triumph.

Justice Samuel Alito, writing for the majority in Harris v. Quinn, found Illinois cannot force taxpayer-subsidized home health care providers to pay union dues.

Pam Harris, the lead plaintiff, argued an Illinois requirement that she pay union dues violated her First Amendment rights. A five-justice majority agreed, overturning lower court decisions.

"If we accepted Illinois' argu­ment, we would approve an unprecedented violation of the bedrock principle that, except perhaps in the rarest of circumstances, no person in this country may be compelled to subsidize speech by a third party that he or she does not wish to support," Alito wrote.