I’m not a political activist. I have nearly two decades of experience in the classroom. And I’ve consistently paid about $1,500 in annual union dues in the belief that my union would use the money to bargain for better wages, fair working conditions and classroom improvements. Instead, the union passed a portion of the dues paid by me and 200,000 other teachers—more than $40 million—to a series of political organizations that backed Mr. Spiller’s failed campaign.

Mr. Spiller finished fifth out of six candidates in the Democratic primary with just 89,472 votes, or 10.6% of the total. That means the union spent about $450 per vote. I and many other teachers pointed out that we were never even surveyed to see if we supported his cause.

Win or lose, the union’s handling of members’ money was wasteful and irresponsible. But I am arguing in a lawsuit filed Sept. 30 in New Jersey Superior Court that it was also an illegal breach of contract and a violation of the union’s fiduciary duty to act in its members’ best interests. On paper, the union tells teachers that giving to its political organizations is voluntary. The membership forms we signed, which function as a contract between a member and the union, have a separate box to check for voluntary donations to the union’s political action committee. I didn’t check that box, and therefore believed my money would not be used by a PAC.

That wasn’t true. In 2013, union officials created an obscure political group called Garden State Forward and funded it with more than $100 million in teachers’ dues. Garden State Forward then passed the money along into Working New Jersey, a Super PAC created last year and led by union insiders past and present, and Protecting Our Democracy, a now-defunct political organization. Both backed Mr. Spiller’s campaign exclusively.

The union leadership led members to believe that donations to its PAC were voluntary. Thousands of teachers signed up for union membership thinking their money wouldn’t be spent on politics. It was a classic bait-and-switch—and it violated the membership contract I signed.

Compounding the problem, Mr. Spiller held decision-making positions at the union and Garden State Forward, and was the chairman of Protecting Our Democracy. New Jersey law also requires union officials to act as fiduciaries who put members’ interests ahead of their own. Mr. Spiller appears to have done the opposite.

Teachers raised concerns. But NJEA officials did nothing to address them adequately. I wrote an open letter to Mr. Spiller questioning his priorities, but he never answered. I have since resigned my membership in protest. Ann Marie Pocklembo, an NJEA member who has been a New Jersey teacher for three decades, posted her objections on Facebook on a members’ group, only to be removed from the group and blocked from the NJEA’s page. When she was later unblocked, a union representative said it could happen again if she continued making those kinds of comments.

Now Ms. Pocklembo and I are suing the union, with the help of the Fairness Center, a nonprofit law firm that represents public employees when they believe unions have violated their rights. Our litigation comes as the New Jersey Policy Institute, a nonprofit policy group, has filed complaints against the NJEA with the Internal Revenue Service and the New Jersey Election Law Enforcement Commission.

The policy institute’s complaint alleges that the union misreported information on its tax filings in a way that disguised the political nature of Garden State Forward and broke IRS reporting rules. It also questions whether the union exceeded the state’s $5,800 per-candidate cap on direct political donations during the gubernatorial race by having two of its related or affiliated groups donate the maximum to Mr. Spiller’s campaign. The group is asking for an investigation into whether the union violated federal tax law and state election law to hide its political spending from teachers and the public.

I believe union officials should play by the rules. And I want my choice to stay out of union politics to be respected. Most of all, I want to hold accountable those who, behind closed doors, benefited themselves at teachers’ expense.