In a 5-4 opinion written by Justice Samuel A. Alito Jr., the court said the home healthcare assistants, some of whom care for their own loved ones, had a constitutional right not to support a union they opposed.

The decision will make it more difficult in some states for unions to continue organizing home healthcare assistants, a rapidly growing segment of the workforce that is expected to double in the next decade thanks to the aging U.S. population. It could also apply to child care workers.

More ominously for unions, the decision practically invites the National Right to Work Foundation, which brought the case, to bring additional legal challenges that could apply the same legal reasoning to millions of other public workers.

“This is what the labor movement fears the worst,” said Gary Chaison, a professor and labor historian at Clark University in Massachusetts. “They’re worried about dying the death of 1,000 cuts. They’re losing the potency of their weapons.”

Beginning in 2003, Illinois officials agreed to designate home-care workers as “public employees” because they care for the disabled and are paid with government Medicaid funds. That cleared the way for the Service Employees International Union to organize them.

Union officials say they have won higher wages and better benefits for 26,000 of the  home-care assistants in Illinois. But anti-union lawyers sued the state, arguing that the personal assistants are not truly public employees and should not be compelled to pay union fees.

About 1 million of the 3 million home healthcare workers nationwide are represented by the SEIU or the American Federation of State, County and Municipal Employees, according to union lawyers. They are mostly in the Northeast, on the West Coast and in some states in the Midwest that, like Illinois, have “fair share” laws requiring people who do not want to join a union to pay limited dues to compensate for the union’s bargaining, which benefits members and nonmembers alike.

But because each state’s law varies, it was not clear Monday whether all of the 10 states with fair share arrangements would be covered by the court’s decision in Harris et al vs. Quinn. Anti-union lawyers said all such laws would probably be ruled unconstitutional.

The decision is “a very serious concern to the unions representing home-care workers,” said Craig Becker, general counsel to the AFL-CIO, which includes AFSCME. “It just makes it more economically difficult by creating incentives for people to not pay their fair share of the cost.” Many home-care workers earned low wages and received no benefits before the union represented them, he said.

Also challenging for the unions is the unusual nature — emphasized by Alito — of the workers involved, who are difficult to organize because they provide care in different homes and have no central workplace. “That’s a serious impediment,” Becker said.

Alito, speaking for himself and the court’s four other most conservative justices, said compelling the home healthcare assistants to pay union dues violated the 1st Amendment.

“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.

That logic also could apply to more conventional public-sector employees, like teachers. Alito has previously made clear that he would support striking down all mandatory union fees as a violation of free speech.

But standing in his way is the 1977 Supreme Court decision in Abood vs. Detroit Board of Education, which said that fair share requirements were constitutional so long as the dues went toward representing workers, and not toward public advocacy, such as political lobbying or campaign contributions.

Labor lawyers said Alito stopped short either because the case did not require him to decide the larger question or because he did not have five votes to do so.

If the former, it suggested that the next shoe could fall soon if another case makes its way to the court.

“I think the opinion certainly makes it clear that Abood is on shaky foundations,” said William Messenger, the National Right to Work lawyer who won the case Monday. “The next challenge to Abood that comes up [to the Supreme Court] has a decent chance of success.”

In one possible test case, 10 California public school teachers who resigned their union memberships sued last year to block a requirement that they pay more than $600 in annual union fees.

Justice Elena Kagan, joined by the court’s three other liberals, welcomed the majority’s restraint in focusing the ruling on home healthcare workers, while at the same time dissenting from the holding on the Illinois workers. Kagan seemed to go out of her way to emphasize that Alito did not have support on the court for overruling Abood.

“Readers of today’s decision will know that Abood does not rank on the majority’s top-10 list of favorite precedents — and that the majority could not restrain itself from saying (and saying and saying) so,” she wrote. “Yet they will also know that the majority could not, even after receiving full dress briefing and argument, come up with reasons anywhere near sufficient to reverse the decision. Much has gone wrong in today’s ruling, but this has not.”

Illinois Atty. Gen. Lisa Madigan, speaking during a conference call for reporters organized by the SEIU, praised the home healthcare program as one that had reduced worker turnover and enhanced stability for those who need help at home. She described the court’s ruling as “a very narrow decision” that allows the SEIU to continue to organize the workers.

Times staff writer Alana Semuels in New York contributed to this report.