CHICAGO — Gov. Scott Walker of Wisconsin, who gave up his quest for the 2016 Republican presidential nomination last month, signed into law on Friday a measure that limits a longstanding tool against political corruption that has been used in investigations of Mr. Walker and his allies.
The John Doe law, as it is called in Wisconsin, has given prosecutors the power to obtain search warrants and order people to testify and turn over documents in investigations that typically take place in secret.
Under the measure, which easily passed both Republican-controlled chambers of the State Legislature, prosecutors will no longer be allowed to use the John Doe law to investigate crimes that include bribery and misconduct in office. The legislation will also limit proceedings to six months and lift an order that barred subjects of an investigation from discussing it publicly.
Prosecutors can still use the John Doe law to investigate violent crimes and drug-related felonies.
Six of Mr. Walker’s aides or allies were convicted as a result of a John Doe investigation. Mr. Walker’s former government office and, later, his campaign were the focus of John Doe investigations of campaign activities and fund-raising, but he was never charged.
Kelly M. Rindfleisch, deputy chief of staff to Mr. Walker during his time as Milwaukee county executive, pleaded guilty in 2012 to felony misconduct in public office after facing charges that she had used county time to perform campaign work.
Republicans said the change in the law was required to curb what they called unnecessarily long and intrusive investigations that amounted to political witch hunts. They said that other tools, like grand juries, could be used to investigate possible political crimes. In a joint statement last month, Robin Vos, the Assembly speaker, and Scott Fitzgerald, the Senate majority leader, said the legislation would allow “sensible changes to the state’s broad John Doe statutes.”
This week, the Assembly approved the bill overwhelmingly, 61 to 36, and the Senate passed it 18 to 14. Both votes fell along party lines.
Democrats denounced the bill as a measure to give politicians cover for committing crimes in office.
Peter Barca, the Democratic minority leader of the Assembly, said last week in anticipation of the bill’s passing, “The era of clean, open and transparent government in Wisconsin is over.”
“I fear for the future of democracy in Wisconsin, and I am not overstating the problem, in my judgment,” Mr. Barca said. Democrats also decried a bill, passed this week, to loosen campaign finance restrictions in the state.
Jennifer Shilling, the Senate minority leader, said in a statement on Friday that the bill to limit use of the John Doe law was “a gross abuse of political power.”
“Republicans should be less concerned about covering up Governor Walker’s political scandals and more focused on helping hardworking Wisconsin families,” she said.
Laurel Patrick, a spokeswoman for Mr. Walker, did not respond to a request for comment.
Jay Heck, executive director of Common Cause in Wisconsin, a nonprofit organization that lobbies for clean and open government and opposed the new legislation, said the John Doe law had allowed district attorneys to successfully prosecute political corruption that might not have been pursued otherwise.
“What this new law does is it exempts from the John Doe process crimes that are committed involving elections, campaign finance and ethics,” he said. “In other words, the crimes that politicians would be most likely to commit. They have carved out a special exemption for themselves.”
A version of this article appears in print on October 24, 2015, on page A13 of the New York edition with the headline: Wisconsin Governor Signs Bill Limiting Political Corruption Inquiries.
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The Principle of Least Interest: He who cares least about a relationship, controls it.
