After much soul-searching, I am filing a civil-rights lawsuit on Wednesday against Milwaukee County District Attorney John Chisholm. I fear his retaliation, given what I know of his methods, but the Chisholm campaign against me that began at dawn on Sept. 14, 2011, requires a legal response to discourage the prosecutor’s continued abuse of his office.

Some background: Mr. Chisholm launched his first round of investigations into the affairs of Scott Walker in May 2010, when he was serving as Milwaukee county executive and Mr....