Kagan agreed with the state's arguments that home care workers should be treated the same as other public workers because Illinois sets their salaries, resolves disputes over pay, conducts performance reviews and enforces the terms of employment contracts.
"Our decisions have long afforded government entities broad latitude to manage their workforces, even when that affects speech they could not regulate in other contexts," Kagan said.
A federal district court and the 7th U.S. Circuit Court of Appeals had rejected the lawsuit, citing the high court's precedent.
Nine other states have allowed home care workers to join unions: California, Connecticut, Maryland, Massachusetts, Minnesota, Missouri, Oregon, Vermont and Washington.