MADISON, Wis. — A Wisconsin Supreme Court justice’s recusal is raising questions about the court’s role in an investigation that conservatives charge is aimed at derailing Gov. Scott Walker’s 2014 re-election campaign — and his presidential prospects in 2016.

Justice Ann Walsh Bradley begged off a case involving a Democrat-launched secret probe of conservative political groups. Three unnamed petitioners are asking the court to stop the politically charged John Doe probe that has spanned more than a year and a half.

Wisconsin’s John Doe law grants grand jury-like powers to judges overseeing the secret probes — without the benefit of a jury of peers.