The Wisconsin prosecutors conducting a secret criminal probe of conservative political groups were warned last month that they were inviting a federal lawsuit. On Monday they got one, as two of their targets charged the prosecutors with violating their civil rights. Sometimes the only way to stop a bully is to punch back.

The lawsuit was filed in federal district court in Milwaukee by David Rivkin and the BakerHostetler law firm on behalf of the Wisconsin Club for Growth and its director Eric O'Keefe. The suit names four Milwaukee...