In a 5-4 decision authored by Justice Samuel Alito, the court created a new class of employee called a "partial public employee" and ruled that they could not be compelled to contribute union dues.

This is a 'splitting the baby' decision that doesn't gut union membership but certainly hobbles it.

I will update after I read the opinion and dissent.

UPDATE:

Justice Alito and the conservative majority spent a lot of time shaking their fists at existing laws before carving out a specific exception for Illinois home care workers. The majority opinion seems to rest on the old 'money is speech' axiom, except in this case they're concerned with workers whose money is used for speech they don't like.

Perhaps the best way to understand the decision is to read Justice Elena Kagan's dissent, which was Kagan at her sarcastic best.

Readers of today’s decision will know that Abood does not rank on the majority’s top-ten list of favorite precedents—and that the majority could not restrain itself from saying (and saying and saying) so. 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: Save for an unfortunate hiving off of ostensibly “partial-public” employees, ante, at 28, Abood remains the law.

On the nature of employees' speech, Kagan writes:

I can see no reason to treat the expressive interests ofworkers objecting to payment of union fees, like the petitioners here, as worthy of greater consideration. The subject matter of the speech is the same: wages and benefits for public employees. Or to put the point more fully:In both cases (mine and the real one), the employer is sanctioning employees for choosing either to say or not to say something respecting their terms and conditions of employment. Of course, in my hypothetical, the employer is stopping the employee from speaking, whereas in this or any other case involving union fees, the employer is forcing the employee to support such expression. But I am sure the majority would agree that that difference does not make a difference—in other words, that the “difference between compelled speech and compelled silence” is “without constitutional significance.”

On agency fees, economics and 'free riders':

The federal workforce, on which the majority relies...provides a case in point. There many fewer employees pay dues than have voted for a union to represent them. And why, after all, should that endemic free-riding be surprising? Does the majority think that public employees are immune from basic principles of economics? If not, the majority can have no basis for thinking that absent a fair-share clause, a union can attract sufficient dues to adequately support its functions.

She concludes with this:

For many decades, Americans have debated the pros and cons of right-to-work laws and fair-share requirements. All across the country and continuing to the present day, citizens have engaged in passionate argument about the issue and have made disparate policy choices. The petitioners in this case asked this Court to end that discussion for the entire public sector, by overruling Abood and thus imposing a right-to-work regime for all government employees. The good news out of this case is clear: The majority declined that radical request.

But also sees the negative impact in this specific case:

The bad news is just as simple: The majority robbed Illinois of that choice in administering its in-home care program.

SEIU:

The ruling places at risk a system of consumer-directed home care in Illinois that has proven successful in raising wages, providing affordable health care benefits, and increasing training. The number of elderly Americans will increase dramatically in the coming years. States need to build a stable, qualified workforce to meet the growing need for home care—and having a strong union for home care workers is the only approach that has proven effective.

NEA:

“As a high school teacher and coach for 23 years, I saw how the entire team benefited when we all worked together. With today’s ruling, the Supreme Court took away the fairness and camaraderie that comes with working in a team. Agency fees are a common-sense, straight-forward way to ensure fairness and protect equity and individual rights. Every educator who enjoys the benefits and protections of a negotiated contract should, in fairness, contribute to maintaining the contract. And fair share simply makes sure that all educators share the cost of negotiations for benefits that all educators enjoy, regardless of whether they are association members.

“Despite today’s decision, we know that public sector workers will continue to organize—in public sector bargaining states and non-bargaining states, in agency fee states and right to work states—because public sector workers know that a union is the best way for all of us to ensure good schools, quality public services and economic prosperity.”

AFT:

"While the court upheld the importance of collective bargaining and unions to families and communities, let’s be clear that working people, who have aspired to the middle class and tried to make a better life for their families, have taken it on the chin for years. Stagnating wages, loss of pensions and lack of upward mobility have defined the economic distress they have experienced. Today’s decision makes it worse.

Corporate money/speech seem to mean something different from employees' money/speech. Imagine this case in the context of shareholders suing a public corporation for making political contributions to candidates those shareholders don't agree with. Do you imagine the majority would rule the same way?