Bob Fioretti was expected to be at the Billy Goat Tavern on West Madison Street Wednesday morning to say he's running as a Republican.
Fioretti previously ran for Cook County Board president and lost to Toni Preckwinkle.
State's Attorney Kim Foxx is not seeking reelection for a third term.
A former appellate court justice launched a bid to replace Foxx in July.
Justice Eileen O'Neill Burke said she wants to bring integrity to the office, and make sure violent criminals are kept behind bars.
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Burke is a former assistant state's attorney and defense attorney who most recently served as an appellate court justice.
Clayton Harris III, a professor of public policy at the University of Chicago, has also announced he's running.
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Richard Boykin said he'll launch his candidacy soon. Others considering running include Jim Durkin and Joe Ferguson.