A dark cloud hanging over Gov. Scott Walker's presidential ambitions was cleared by a state Wisconsin Supreme Court ruling Thursday morning.

In a 4-2 ruling, the court ordered an end to the John Doe investigation into coordination between Walker's campaign and conservative groups that supported him during his 2012 recall election. 

The John Doe investigation into suspected illegal coordination between Walker's recall campaign and conservative issue advocacy groups, launched by Milwaukee County District Attorney John Chisholm in 2012, was put on hold more than a year ago.

In its ruling, the court said outlawing such coordination is "unconstitutionally overbroad and vague under the First Amendment."

"To be clear, this conclusion ends the John Doe investigation because the special prosecutor's legal theory is unsupported in either reason or law. Consequently, the investigation is closed," Justice Michael Gableman wrote in the court's majority opinion.

In a concurrent opinion, Justice David Prosser wrote that the search warrants and subpoenas issued as part of the investigation were "so broad and so extensive that they make the fruits of the legendary Watergate break-in look insignificant by comparison."

Todd Graves, who with Edward Greim served as counsel to the Wisconsin Club for Growth and Eric O'Keefe, said prosecutors in the case ignored protections afforded by the Bill of Rights.

"They used search warrants to conduct pre-dawn raids on families and secretly obtained millions of personal emails from numerous parties, including individuals who still do not know they were targeted," Graves said in a statement. "They counted on a veil of secrecy to assault the fundamental liberties of our clients and commit taxpayer funds for an outrageous misuse of their offices and the law. They used their unlawful investigation to try to threaten and embarrass groups they do not like, specifically because of the groups’ constitutionally protected efforts on libertarian and conservative issues. Until today, District Attorney John Chisholm and the GAB acted like playground bullies without fear of restraint from the courts."

Through the decision, the court ordered that the special prosecutor and the district attorneys involved in the investigation "must cease all activities related to the investigation, return all property seized in the investigation from any individual or organization, and permanently destroy all copies of information and other materials obtained through the investigation."

The decision comes just days after Walker's official announcement that he'll seek the Republican nomination in 2016. That campaign draws heavily from a theme of "recalling the recall," which was spurred by Walker's controversial Act 10 legislation. The bill curbed collective bargaining rights for most public employees, making Walker enemy number one for unions and sparking massive protests at the Capitol.

By ending the probe, the court removes a potential vulnerability from Walker's presidential campaign. But critics of the ruling fear it will have lasting negative effects on electoral politics in Wisconsin.

"Lest the length, convoluted analysis, and overblown rhetoric of the majority opinion obscure its effect, let me state clearly: The majority opinion adopts an unprecedented and faulty interpretation of Wisconsin's campaign finance law and of the First Amendment," wrote Justice Shirley Abrahamson in her dissent. "In doing so, the majority opinion delivers a significant blow to Wisconsin's campaign finance law and to its paramount objectives of 'stimulating vigorous campaigns on a fair and equal basis' and providing for 'a better informed electorate.'"

Abrahamson accused the majority opinion of disregarding both the letter and spirit of the state's campaign finance laws.

"This, within the realm of issue advocacy, the majority opinion's theme is 'Anything Goes,'" Abrahamson wrote.

Critics questioned whether some justices should have weighed in on the case at all. The liberal group One Wisconsin Now had previously called for the four judges who formed the majority to recuse themselves, because they had received more than $8 million in campaign donations from parties named in the investigation.

“It’s clear that one party rule in the legislative, executive and judicial branches in Wisconsin means Scott Walker and his cronies are free to run amok, ignoring the rules whenever it gives them a partisan advantage and looting the state for their campaign benefactors," said OWN executive director Scot Ross. "If this exact scenario were occurring in another country, Scott Walker would be calling for boots on the ground to save democracy."

At the same time, groups like the Wisconsin Democracy Campaign called the ruling "downright dangerous," warning that it would "make a mockery of campaign contribution limits."

Jay Heck, executive director of Common Cause in Wisconsin, called for the case to be appealed to the U.S. Supreme Court.

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"Unless it is successfully appealed, the decision could conceivably now lead to unlimited coordination between outside spending groups with candidate committees and effectively render contribution limits meaningless in Wisconsin," Heck said.

But conservatives, who decried the second investigation as a partisan witch hunt that has effectively silenced political groups from exercising free speech, were vindicated by the ruling.

"I really think this is a victory for everyone. It just has always struck me as extremely disturbing that you would .... essentially have prosecutors of one party conducting a four-year investigation into the political apparatus of the other side," said Rick Esenberg, president and general counsel of the conservative Wisconsin Institute for Law & Liberty. "Even if it's done with the best of intentions, it's not the way we should conduct politics in this state."

Esenberg said it's now time for the Legislature to revisit the section of state law governing campaign finance restrictions. 

"The Wisconsin Legislature must move quickly to enact laws that protect and affirm the free speech rights of independent groups and others, and to ensure that the prosecutorial and regulatory abuses evident in this John Doe investigation never happen again in our state," said Wisconsin Manufacturers and Commerce president and CEO Kurt Bauer in a statement. "Taxpayers deserve to know how much of their money was wasted on this unlawful and unconstitutional investigation, and the targets deserve an apology from the prosecutors and GAB regulators who have violated their rights."

Responding to critics, Esenberg said the U.S. Supreme Court is unable to review a state Supreme Court's determination of state law. It could review the four judges' decisions not to recuse themselves, but he said that would be "highly unlikely." 

"This closes a divisive chapter in Wisconsin history, and the assertive recognition of First Amendment rights by the Wisconsin Supreme Court protects free speech for all Wisconsinites," said Attorney General Brad Schimel in a statement. "The Court’s decision leaves no doubt that the John Doe investigation is over."

A prior John Doe investigation, also launched by Chisholm, resulted in convictions for six Walker associates during his time as Milwaukee County executive.

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Jessie Opoien covers state government and politics for the Capital Times. She joined the Cap Times in 2013 and has also covered Madison life, race relations, culture and music. She has also covered education and politics for the Oshkosh Northwestern.