MADISON, Wis. (AP) – Two unions representing about 2,700 public workers in Madison and Dane County have filed a federal lawsuit challenging the constitutionality of Wisconsin’s new law restricting collective bargaining rights.

The lawsuit, filed Wednesday in U.S. District Court, is the second federal challenge to the law that was pushed by Republican Gov. Scott Walker and passed by the GOP-controlled Legislature earlier this year. The other challenge, filed last month by a broad coalition of unions, is pending.

The law prohibits collective bargaining except over base salary increases no greater than inflation. It took effect June 29 after the state Supreme Court rejected a lawsuit challenging how the bill passed the Legislature.

The law applies to most public workers, including school teachers, but does exempt most police and firefighters, local transit workers and emergency medical service employees.

Exempting some public workers from the law is a violation of equal protection rights under the U.S. Constitution and creates unnecessary conflict, argues the lawsuit filed by two local Madison chapters of the AFL-CIO. The lawsuit, which asks the court to block further implementation of the law, argues it also violates First Amendment rights under the Constitution.

A spokesman for the state Justice Department, which will defend the case, had no immediate comment Thursday. The attorney representing those who filed the lawsuit was out of the office and did not immediately return messages.

The lawsuit argues that limitations on collective bargaining impair the ability of public sector labor unions to survive and to express the views of those they represent.

The law limits the terms of collective bargaining agreements to one year and prohibits any extensions, requires unions to vote every year to stay in existence, and disallows the automatic withdrawal of union dues from employees’ paychecks.

The lawsuit alleges that those restrictions also impair the ability of public workers to associate with labor unions and to express concerns of mutual interest with their employers.

There is no constitutionally acceptable governmental interest in imposing the restrictions, the lawsuit said.

The other federal lawsuit makes similar arguments. Groups suing in that case include councils of the American Federation of State, County and Municipal Employees, the Wisconsin State AFL-CIO, the American Federation of Teachers, the Wisconsin Education Association Council, the Wisconsin State Employees Union and the Service Employees International Union-Health Care Wisconsin.