Rep. Mark Kirk has opted to skip the forum, but has not responded to requests for explanation.
Scheduled to attend are Republicans Andy Martin, Kathleen Thomas, John Arrington and Patrick Hughes.
The debate is among several airing on ABC 7 Chicago in a partnership with the League of Women Voters and the Better Government Association.
The Democratic Senatorial Campaign Committee was quick to slam Kirk over his absence.
"Kirk, who has many questions to answer due to his numerous campaign flip-flops, hypocrisies, and distortions, continues to do Illinois Republicans a disservice by refusing to compare his record to the other candidates in the public arena," the DSCC said in a statement.
Some are theorizing that Kirk's absence may have something to do with the invitation extended to Martin, a perennial political candidate with a history of anti-semitism and legal troubles. In a political ad last month, Martin made claim that Kirk is gay and is part of a "Republican Party homosexual club."
Kirk denied the allegation and the Illinois Republican Party disavowed Martin's ad and said that he was no longer recognized "as a legitimate Republican."
Full Coverage: Illinois Elections 2010