The Chicago Bar Association says it will ask the Illinois Supreme Court to examine the process that put a constitutional convention referendum on the ballot.
The Illinois Constitution says that every 20 years it's the secretary of state's job to put the convention question on a separate ballot.
Attorney Steven Pflaum says those rules weren't followed this time around.
He says the court could order a do-over of the failed referendum election.
Meanwhile, supporters of the referendum are gathering in Chicago to discuss whether they should sue over what they called an unfair election.
State elections officials didn't respond immediately to requests for comment by the Associated Press.