By Brendan O’Brien and Mary Wisniewski

MILWAUKEE/CHICAGO (Reuters) – Wisconsin prosecutors asked a federal appeals court on Tuesday for approval to restart an investigation into possible illegal coordination between Governor Scott Walker’s campaign and special interest groups during elections in 2011 and 2012.

A federal judge in May halted the investigation after a lawsuit by the Wisconsin Club for Growth, a conservative organization, accusing investigators of sidelining it from political activities, violating its rights to free speech, association and equal protection.

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Prosecutors appealed the judge’s ruling shutting down the probe launched in 2012 by Milwaukee County Democratic District Attorney John Chisholm.

A three-judge panel of the 7th Circuit U.S. Court of Appeals took the case under advisement on Tuesday after hearing arguments in Chicago by lawyers for the special prosecutors and the Wisconsin Club for Growth.

Prosecutors argued they had immunity from being sued in court and said in court papers that the interests of the public outweighed those of an interest group.

Neither Walker, a potential Republican White House hopeful in 2016, nor anyone else has been charged in the investigation, which has been pursued under a Wisconsin law that requires such probes to be conducted in secret.

Walker was elected governor in 2010. In June 2012, he became the first governor in U.S. history to survive a recall election. The investigation was launched two months later.

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Walker was forced into a special election after he angered Democrats and labor unions by pushing through the state Legislature a bill stripping public-sector labor unions of much of their power.

In previously released court documents, prosecutors suggested Walker’s campaign advisers gained control over both the funding and strategy of independent conservative groups, in what could be a violation of campaign finance laws.

Walker and Democratic challenger Mary Burke are in a statistical dead heat in opinion polls less than two months before the November governor’s election.

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(Editing by Eric Walsh and Peter Cooney)