CHICAGO (AP) — A voter referendum aimed at changing the way Illinois draws its political boundaries is unconstitutional for the November ballot, a judge determined Wednesday.

The ruling by Cook County Circuit Court Judge Diane Larsen marked the second time that an attempt to change how Illinois redistricts has been blocked in the courts. An attorney who has represented the state's Democrats, including House Speaker Michael Madigan and Senate President John Cullerton, filed a lawsuit claiming it wasn't constitutional.

The plan by a group called the Independent Map Amendment proposed an 11-member commission to take over how legislative boundaries are drawn in the state, instead of leaving it to party leaders. Commission members would be chosen through a complex process involving the state's auditor general and potentially members of the Illinois Supreme Court. Backers of the measure, who collected nearly 560,000 signatures across the state, said they learned from a failed 2014 effort to overhaul redistricting.

The group vowed to file an expedited appeal to the Illinois Supreme Court.

"A great deal of care went into crafting an amendment that follows constitutional guidelines while also creating a system that is independent, fair, transparent, and protects the ability of minority communities to elect candidates of their choosing," Independent Maps chairman Dennis FitzSimons said in a statement.

"Redistricting reform was specifically addressed by the framers of our constitution as a 'critical' area for citizen petition initiatives. We believe that the Illinois Supreme Court will side with Illinois voters and not deny citizens the opportunity to vote on this amendment."

The lawsuit claimed that the measure didn't meet the narrow scope that changes to the state Legislature be both "structural and procedural." He filed the lawsuit on behalf of minority business and community groups who call themselves the People's Map and say the current mapping process protects minority representation. Those arguments were never mentioned in court. A People's Map spokesman didn't immediately have comment on Wednesday.

In court, Independent Maps' lawyers countered by saying since legislative districts are the "building blocks of the General Assembly," redistricting is by its nature structural and procedural.

The Illinois State Board of Elections had already determined that the group had enough valid signatures for the ballot.

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Follow Sophia Tareen at http://twitter.com/sophiatareen .