MADISON, Wis. — Eric O’Keefe knows the consequences of speaking out against powerful prosecutors in a politically charged investigation.

“Indeed, Mr. O’Keefe has faced threatening behavior — including online stalking of his children and grandchildren — due to his participation in this litigation,” the conservative activist’s attorney writes in a court document filed this week.

O’Keefe, the man who sued Milwaukee County prosecutors of a secret John Doe probe into dozens of conservative groups and has won some key battles in the name of the First Amendment, has learned the risks of taking such a stand.