On Thursday, September 10, Illinois held a special U.S. House election to fill the vacancy in the 18th district. According to this news story, Darin LaHood, the Republican nominee, received 34,907 votes; Robert Mellon, the Democratic nominee, received 15,840 votes. That totals only 50,747 votes, although when the results are official, they will probably be slightly higher.
The last time Illinois held a special U.S. House election, on April 9, 2013, to fill the vacant 2nd district seat, there were six candidates on the ballot: one Democrat, one Republican, one Green, and three independents. The turnout in that election was 83,193 votes cast, significantly better than the September 2015 race. The reason there were alternate candidates on the ballot in 2013 is that a U.S. District Court Judge had struck down the 5% petition requirement for independent candidates and the nominees of unqualified parties, as applied to special elections. That case was Jones v McGuffage, a Green Party ballot access victory. Despite the 2013 court victory, Illinois election officials continued to require 5% petitions in the September 2015 race, and no one went to court in 2015 to change that.