One of the 11 Democratic state senators who fled Wisconsin for Illinois during the 2011 statehouse showdown over Gov. Scott Walker's union reforms must disclose emails related to the legislative battle, a state appeals court ruled Wednesday morning.

Democratic State Sen. Jon Erpenbach has been fighting an effort by the John K. MacIver Institute for Public Policy, a free-market think tank, to use the state's disclosure laws to release the emails. He has refused, saying the names and address of his correspondents should remain private.

A unanimous three-judge panel rejected that claim: "Transparency and oversight are essential to honest, ethical governance. Erpenbach has not met his burden of establishing that the public interest in nondisclosure of the redacted information outweighs the significant public interest in disclosure."

Brett Healy, president of the MacIver Institute, said the ruling was a "win for transparency and the taxpayers," adding: "Government employees and their unions should not be given a cloak of anonymity to hide their attempts to influence the legislative process. Today's decision requires them to play by the same rules as everyone else."

Erpenbach released a combative statement declaring that he "never regret standing up for the constitutional rights of people to contact their elected leaders." He did not say if he would continue to fight the court's ruling, though, instead saying he would "weigh all of my legal options."

The case dates back to a March 2011 request from the institute asking Erpenbach to release the emails. The lawmaker provided only redacted versions, claiming that names and addresses from nongovernment accounts were not subject to the state's sunshine law. The redactions were poorly done, though, and it was evident that many of the emails came from addresses ending in .gov, which is exclusive to public officials. Therefore, they would fall under the disclosure law, the institute has argued.

"Based on a review of the emails, the MacIver Institute concluded that there was a concerted effort on behalf of government employees to actively use public resources to campaign against Wisconsin's collective bargaining reforms known as Act 10," the institute said.

Act 10 is the official name for Walker's reforms, which allowed state and local government workers to quit their unions if they wanted. It also ended automatic union dues deductions for those workers, limited collective bargaining to wages and required the unions to submit to annual recertification votes by their members. Labor organizations fought the rules, fearing that without coercive contracts that required employees to be in a union, many would leave. Many did, though union membership grew again last year.

Erpenbach, who represents a suburban Madison district heavily populated by public employees, was a key figure among the fleeing Illinois Senate Democrats. The senators left the state on Feb. 17, 2011, and returned on March 12, 2011, in an effort to deprive the Republican majority of a quorum to pass the bill. During that period, Erpenbach served as their primary spokesman.

Once in Illinois, the senators "worked closely with union officials," particularly the American Federation of State, County and Municipal employees, according to More Than They Bargained For: Scott Walker, Unions and the Fight for Wisconsin, a 2013 account the legislative battle written by Milwaukee Journal Sentinel reporters Jason Stein and Patrick Marley. The union even provided the senators with talking points for their media interviews.

The senators became a cause célèbre for liberal activists, but the gambit failed when statehouse Republicans simply tweaked Act 10 enough that it didn't require the quorum to pass under Senate rules.