The Wisconsin Supreme Court put a stop Thursday to a probe by a group of prosecutors alleging collusion between Gov. Scott Walker's political operation and nonprofit groups as Mr. Walker fought a 2012 recall effort, the Milwaukee Journal Sentinel and The Christian Science Monitor report.
The 4-2 ruling could reshape Wisconsin politics by seemingly authorizing cooperation between campaigns and advocacy groups that are not required to disclose donors.
The "John Doe" investigation, so-called because parties involved were barred from speaking publicly about the case, focused on whether aides to Mr. Walker directed political activity by outside entities such as Wisconsin Club for Growth, a conservative nonprofit. Supporters of Mr. Walker, a Republican presidential candidate, branded the investigation a political witch hunt.
Writing for the majority, Justice Michael Gableman said the activities targeted by prosecutors were not illegal and called state's campaign-finance laws "unconstitutionally overbroad and vague."