A Seattle branch of the Service Employees International Union saw almost half of its members walk away in a single year following a 2014 Supreme Court decision that gave those workers that right for the first time.

The sharp decline underscores why public-sector union leaders opposed the court's decision in Harris v. Quinn: In many cases, large numbers of their members were not interested in being in a union in the first place.

Local 925's membership, mainly state-subsidized family child care providers, fell from 6,600 in June 2014 to 3,700 in May 2015, according to Washington State's Department of Social and Health Services. That followed the court's ruling in Harris v. Quinn that Illinois state-subsidized in-home caregivers for the disabled were not public-sector employees and therefore not eligible for collective bargaining.

While technically limited to Illinois, the ruling had a cascading effect in other states such as Washington, Massachusetts and Minnesota as public-sector unions moved to head off any potential legal challenges that would use the court's ruling. The main response has been to allow workers to drop their membership if they request it. That way, the union can at least keep the remaining members who do wish to belong and maintain their contract with the state as the workers' representative.

In August 2014, Washington stopped automatically deducting dues from the paychecks of Local 925 members who had never signed up to be a union member. Those people could still support the union if they wanted, but they would have to sign up and request that the state make the deductions.

"Only about a dozen members had registered objections to automatic dues deduction prior to the ruling," said Maxford Nelson, labor policy analyst at the conservative Freedom Foundation, which obtained the Department of Social and Health Services data. "Then, a third of their membership, 38 percent, disappeared overnight."

Union membership continued to decline from there, partly because the foundation embarked on a public relations campaign to alert the union members who had authorized dues deduction that they could get the deductions suspended if they requested.

The union argues that most of these workers just hadn't gotten around to signing up yet.

"Our union made the decision to no longer require fair-share fees from family child care providers in 2014," SEIU Local 925 President Karen Hart told the Washington Examiner. "Only a very low percentage of providers have actually opted out of our union. New providers who come into the profession, or are simply taking care of a friend's or family member's child, have simply not had a chance to join yet. They are represented by our union and receive union wages and benefits from day one, even though they may not have joined our union yet."

Over the last decade and a half, SEIU and other unions have prodded Democratic-leaning states to declare that various state-subsidized in-home caregivers are public-sector employees and therefore eligible for collective bargaining. The unions then have pushed the states to make them the workers' representative, taking a cut of their state subsidy checks to pay for union dues.

Many of the caregivers are people taking care of elderly or disabled family members who often don't realize that by signing up for the subsidies they are becoming union members. Several such caregivers were the plaintiffs in Harris v. Quinn.

Another Washington SEIU branch, Local 775, which represents home healthcare workers, did not face the same kind of losses. State data shows that its membership, about 33,600 at the time of the ruling, is now slightly higher at 33,700. A representative of Local 775 did not respond to a request for comment.

Nelson said the difference was because the state is continuing automatic dues deduction even for members who haven't signed up, though it is allowing members to voluntarily opt out if they do so in writing. Nelson contends that few members of Local 775 know this. The union has not made a point of alerting members of their new right and it has legally blocked the foundation from getting the members' names under the state's public records law, preventing the kind of PR campaign that cut Local 925's membership.

A spokesman for Department of Social and Health Services did not respond to a request for comment.