The judge overseeing the probe, Gregory Peterson, granted a request from Club for Growth members in January to quash five subpoenas. The special prosecutor leading the investigation, Francis Schmitz, has asked the 4th District Court of Appeals to review Peterson's decision. Republican Gov. Scott Walker's campaign attorney has asked the Supreme Court to take the case directly.
The court is controlled by a four-justice conservative majority. The group One Wisconsin Now sent a letter to the high court Monday asserting that Club for Growth and Wisconsin Manufacturers and Commerce have spent heavily to support the four justices' campaigns.