The demise of the IL GOP happened after 1994. 1998 was the only year, during the past 20 years, during which Illinois elected a Republican to the U.S. senate and a Republican to be governor, during the same election. Those candidates were Peter Fitzgerald and George Ryan, respectively. Conservatives must have lost power before that election, since Ryan is pro-choice and pro-gun control.
I think the rot began to set-in well before that, and is directly related to the duration of the Thompson administration. The Repubs simply became soft.
In 1998, one gubernatorial candidate favored reduced spending and concealed carry, and was pro-life. The other candidate was an anti-gun big-spender who claimed to be pro-life but didn't have the record to back it up.
The Republican won!?