From Ballot Access News:

In early 2015, just before outgoing Illinois Democratic Governor Pat Quinn left office, he signed a bill that mandates election-day registration at the polls in counties and cities with a population greater than 100,000. The bill had passed on a party-line vote, with all Democrats voting “yes” and all Republicans voting “no.”

On August 4, 2016, Republican U.S. House nominee Patrick Harlan, running in the 17th district, filed a lawsuit against the new law, arguing that it discriminates against voters in small-population counties. Harlan v Scholz, n.d., 1:16cv-7832. The complaint points out that all the other states with election-day registration apply that policy equally to the entire state.

The main defense of the state to the lawsuit seems to be that Harlan should have filed the lawsuit sooner. The law allows small-population counties and cities to use election-day registration, but does not mandate it for small-population jurisdictions, and most of the smaller places are not implementing election-day registration at every polling place. However, the law does mandate that every jurisdiction provide somewhere in the county to register on election day.

Harlan wants an injunction stopping the new law from going into effect. The case is before U.S. District Court Judge Samuel Der-Yeghianyan, a Bush Jr. appointee, who could rule at any time.