Keyes was picked by state GOP committeemen in Chicago late Wednesday.
Jack Ryan, winner of the March primary, withdrew six weeks ago after a judge unsealed divorce filings alleging that he tried to force his wife to go to sex clubs. Former Bears coach Mike Ditka declined to run, and another potential candidate was dropped because of allegations that she harassed a subordinate while she was working for the Bush administration.
Keyes, 53, lives in Maryland, outside Washington, and would have to move to Illinois to run. Crain's cited sources among Illinois Republicans who say Keyes' candidacy is a "done deal" and that he will appear in Schaumburg, Ill., to begin the race.
Keyes has promised the party his answer Sunday after taking a few days to consider a late Senate run against rising Democrat Barack Obama.
Obama, 42, said he would welcome Keyes to the race and hoped the campaign would focus on issues like affordable healthcare and education. Both men are Harvard-educated polished debaters and African-American. Obama has a $10 million campaign war chest while Keyes would need party assistance in fundraising.