The Supreme Court dealt a limited blow to organized labor Monday by ruling that some government employees did not have to pay any fees to the unions representing them. But the court declined to strike down a decades-old precedent that required many public sector workers to pay union fees.

Writing for the majority 5-4 opinion, Justice Samuel A. Alito Jr. concluded that there was a category of government employees — a partial public employee — who can opt out of joining a union and not be required to contribute union fees.

Alito wrote that home-care aides who typically work for an ill or disabled person, with Medicaid paying their wages, should be classified as partial public employees and should not be treated the same way as public schoolteachers or police officers who work directly for the government.

The court’s decision, on behalf of the five most conservative justices, was a partial, but not total win, for labor’s critics. And while labor sustained a defeat in this ruling, it did not amount to a crippling loss that unions had feared.

If the court had overturned the precedent requiring many government workers to pay union fees, it could have greatly reduced the membership and treasuries of public-employee unions. Several legal experts said Alito evidently had tried unsuccessfully to obtain the needed votes for a broader decision to overturn that precedent.

Alito wrote that unions played such a limited role for “partial public employees” like home-care aides that these aides should not be required to pay union fees. Indeed, he wrote that such a requirement would violate their First Amendment rights. He noted that states often set wages for these workers and that unions often did not bargain for them.

The case, Harris v. Quinn, was brought by eight Illinois workers who provided home care to Medicaid recipients. Several plaintiffs were mothers who, helped by Medicaid, were personal assistants to their disabled children and opposed joining the union and paying union fees.

The plaintiffs asked the court to overrule a 1977 decision that declared that government employees could be required to pay fees to unions for representing them and administering their contracts even if they disagreed with the unions’ positions. The majority declined to overrule that foundational decision, Abood v. Detroit Board of Education, although Alito voiced strong discomfort with it.

Objecting to the so-called agency fees that the Abood ruling said teachers must pay, Alito wrote Monday, “Agency-fee provisions unquestionably impose a heavy burden on the First Amendment interests of objecting employees.”