A tie vote from the Supreme Court means public sector unions in about half the states can continue collecting fees from workers who choose not to join.
The justices on Tuesday divided 4-4 in a case that considered whether public employees represented by a union can be required to pay "fair share" fees covering collective bargaining costs even if they are not members.
The split vote leaves in place an appeals court ruling that upheld the practice.
The result is an unlikely victory for organized labor after it seemed almost certain the high court would rule 5-4 to overturn a system in place nearly 40 years.
The court is operating with only eight justices after the death of Justice Antonin Scalia, who had been expected to rule against the unions.