Article 1, Section 2, Clause 3, of the U.S. Constitution requires legislative districts to be based upon an actual count of population every 10 years.  The Illinois Constitution also requires this.

The procedure for drawing legislative districts prescribed by the Illinois Constitution is for the Legislature to complete a redistricting plan for legislative districts according to the new federal census, in the year following the census.

If the redistricting plan is not complete by June 30th, a commission of 8 members (half to be non-legislators), appointed by July 10^th by each of the majority and minority political leadership, “no more than four of whom shall be members of the same political party,” will complete the redistricting plan.… Continue Reading