Anti-union groups are making another major push in the U.S. Supreme Court to eliminate mandatory union dues, so-called “fair share” fees, for millions of public sector workers. This time, a full bench—if it takes the case—could end the deadlock that frustrated their efforts last year.

The latest attempt—Janus v. AFSCME—comes on the heels of the court's 4-4 divide in a nearly identical case—Friedrichs v. California Teachers Association. Before the death of Justice Antonin Scalia, the unions appeared headed for defeat in Friedrichs—with Scalia a likely vote against them. The views on the issue of Justice Neil Gorsuch, Scalia's successor, are unknown, but he may well hold the key to whether agency shop fees survive.

The stakes are high for unions. Agency shop fees are charged non-union members to cover the costs of collective bargaining by unions serving as the exclusive representatives of a workplace's employees. Political activities are excluded.

The filing of the Janus case, brought by the National Right to Work Foundation, is no surprise. Friedrichs itself was carefully crafted by the Center for Individual Rights to respond to recent hints from Justice Samuel Alito Jr. Alito used two decisions to indicate that he believed a landmark union-fee case—Abood v. Detroit Board of Education—violated the First Amendment and should be overturned. The Abood case, decided in 1977, provided the constitutional underpinnings for the union fees. The Supreme Court has reaffirmed Abood at least four times in the last 40 years.

If the high court passes on Janus, the National Right to Work Foundation has more challenges pending involving public sector workers in Pennsylvania, Kentucky, Massachusetts, California, New York and Connecticut. Here are six key questions about the latest attempt from union foes to challenge shop fees.

How is the new case any different from the California teachers union dispute?

There are a number of differences, but none is major in substance. First, the parties asking the justices to hear the case are different. Mark Janus, a child support specialist for the Illinois Department of Healthcare and Family Services, is the lone petitioner. In the Friedrichs case, Rebecca Friedrichs, a California public school teacher, was joined by nine other teachers and the Christian Educators Association International.

Second, the unions being challenged also differ. In Janus, the union is the American Federation of State, County and Municipal Employees, Council 3. Friedrichs named the California Teachers Association. The California attorney general intervened and was a respondent in the Supreme Court.

Third, the political organizations pushing the challenges forward also differ. The National Right to Work Legal Defense Foundation and the Liberty Justice Center are behind the Janus petition. The D.C.-based Center for Individual Rights initiated the Friedrichs case.

The new case urges the justices to overrule Abood and to hold that public-sector agency fee arrangements violate the First Amendment. The Friedrichs case urged the same results, but also asked the justices to hold that the First Amendment is violated when public employees are required to opt out of fees for nonchargeable union activities, rather than affirmatively consent to them.

What are the chances the Supreme Court takes the case?

When the justices heard arguments in the Friedrichs challenge in March 2016 with Scalia on the bench, the outlook looked grim for the unions. Because the court gives no indication of why or on what issue it deadlocks in a 4-4 affirmance, it is only clear that four justices—most likely the four conservatives judging from their comments during arguments—were ready to vote against the union's position.

Those four are still on the court and if they still feel strongly, they may well vote to take the Janus challenge, especially if they believe they can find a fifth vote to decide the case. So, yes, the chances are good that the court will take the case. Only four votes are needed to grant review.

On the other hand, some justices may be reluctant to jump back into the issue so soon after a 4-4 deadlock, especially if the result would be to overturn a 40-year-old precedent with a new justice making the difference. The optics are not good because the court would be viewed by some in the public as a political—partisan—instrument.