Three Anoka County residents are suing the state's largest school district and its teachers union over what they allege are illegal public subsidies of union political activities.

The suit filed last week in Anoka County District Court is being led by veteran advocates of conservative causes and focuses on how teachers are paid while on leave helping to tend to union business.

Those activities include door-knocking and other political campaign efforts, according to the Upper Midwest Law Center, which sued on behalf of the three residents. The organization works to limit "public union overreach" and has been active in helping nonunion public-sector workers opt out of contributing to their unions.

According to the complaint, the Anoka-Hennepin School District has an agreement with Anoka Hennepin Education Minnesota that allows teachers 100 days of leave per year to work with the union. The district pays those teachers a salary ranging from $227 to $562 per day, but the union reimburses only the cost of a substitute teacher, which is $135 to $145 per day, the complaint states.

"Minnesota law is clear: government employers cannot provide benefits to unions without full compensation, and unions cannot force government employers to subsidize union business," Doug Seaton, the law center's president, said in a news release announcing the lawsuit.

In an interview, Seaton said he did not know how much the "subsidy" might add up to in a given year.

Val Holthus, president of the union local, said students need educators to advocate for them. The lawsuit, she said, would make it harder for them to negotiate for better learning and working conditions in the schools and to talk with parents and other decision­makers about what children need to thrive.

She said that the system of leaves agreed to by the union and the district recognizes that the two sides often need to talk when classes are in session, and educators occasionally have to meet with state legislators on their time, too.

"Our union is committed to doing all we can to prepare all our students for successful lives," Holthus said. "This latest attack on our educators' freedom to work in union for the good of our students and our district won't change that."

Don Huizenga, one of the residents suing the district, also was part of an unsuccessful effort to have an Anoka-Hennepin brochure about 2011 school-levy ballot questions declared as promotional and not informational in nature. He said in the news release that his tax dollars should not be used to pay for the union's "political speech."

The most recent lawsuit asks that the union pay the district for all under-reimbursed leave for at least the past six years. Seaton said it's hoped it also will encourage other districts to drop any similar arrangements.

"We thought this was a good opportunity to challenge this," he said.

The district said this week it had yet to review the details in the complaint.