The lawsuit, by the Wisconsin Club for Growth and director Eric O'Keefe, was filed in U.S. District Court in Milwaukee against federal prosecutors and the judge overseeing the ongoing John Doe probe.
The current investigation grew out of a previous one that lasted three years and ended in 2013 with six convictions, including three former Walker aides. The latest investigation, which began in August 2012, is reportedly looking into activity by Walker's campaign, the Club for Growth and more than two dozen other conservative independent groups that were heavily involved in recall elections targeting Walker and state senators.
Walker became a national conservative hero, and possible 2016 presidential candidate, after he became the first governor in U.S. history to win a recall election in 2012. He was targeted for recall because of the law he championed, in the face of massive protests, that effectively ended collective bargaining for most public workers.
Nearly $81 million was spent by the candidates, special interest groups and political committees in that race, more than doubling the previous record set by Walker in the 2010 election.
Wisconsin Club for Growth spent an estimated $100,000 on the recall, according to the nonpartisan Wisconsin Democracy Campaign. The Club for Growth and O'Keefe argue in the lawsuit that the investigation is a violation of their free speech rights as well as their constitutional right to free assembly and equal protection.
"This secret investigation and gag order on conservative activists is intended to stop their political successes in Wisconsin," O'Keefe said in a statement. "The state cannot be allowed to silence political speech it does not like."
O'Keefe and Club for Growth argue in the 76-page complaint, which has many sections and entire pages blacked out, that the secret investigation unfairly targets Walker supporters with "intimidating behavior and harassment to achieve the goals of their politically motivated quest."
"These extraordinary circumstances call for extraordinary action from the federal judiciary," said the lawsuit filed by O'Keefe's attorney, David Rivkin, of Washington, D.C.
John Doe investigations are similar to federal grand jury inquiries in that testimony and material can be gathered secretly. Those involved are also barred from speaking publicly about the proceedings, but O'Keefe has revealed details about it to the Wall Street Journal.
The lawsuit doesn't reveal much that wasn't already known, but is another sign of how aggressively the probe is being opposed.
The lawsuit asks the court to issue injunctions to stop the investigation and award damages to O'Keefe and Club for Growth as determined following a jury trial. It names as defendants Reserve Judge Gregory A. Peterson, special prosecutor Francis Schmitz, Milwaukee District Attorney John Chisholm, Assistant District Attorney Bruce Landgraf, Assistant District Attorney David Robles and investigator Dean Nickel.
Peterson, Schmitz and the district attorney's office did not immediately return messages seeking comment. Nickel had no listed telephone number.
The latest filing comes after three unnamed petitioners filed a lawsuit in November asking the state Court of Appeals to stop the investigation that's taking place in Milwaukee, Iowa, Dodge, Dane and Columbia counties. But last month the appeals court allowed the probe to continue and released documents showing the investigation has five targets and centers on campaign finance laws.
Documents collected in the earlier John Doe investigation related to a former Walker aide are about to be made public. Kelly Rindfleisch, who worked as Walker's deputy chief of staff in his Milwaukee County executive office, pleaded guilty in 2012 to doing campaign work on government time.
A state appeals court judge last week ordered that thousands of pages of emails and other documents collected in her case be made public, after The Associated Press and other news organizations asked those be released.
Some of the documents relate to Rindfleisch's work for Republican Brett Davis's 2010 run for lieutenant governor and likely will include communications with Walker and his campaign team.
Rindfleisch's attorney, Franklyn Gimbel, sent a letter to the judge Friday saying he does not believe the records should be released, but that Rindfleisch will not take the 30 days allotted to review them for private information.
On Monday, Gimbel said Rindfleisch would also not ask the Supreme Court to review the ruling, clearing the way for the records to be released.