The last GOP presidential win in IL was Bush in 1988. Dukakis only had a margin of 250 K in Cook and that was not enough to carry him statewide as he lost by 100 K, even though there were still a lot of southern IL counties voting D.
Clinton had a 640 K margin in Cook in 1992 and that was most of his margin on 720 K that year. 1996 was similar with a 690 K margin in Cook and 750 K statewide. During this decade the inner townships of Cook really started to tilt D creating margins that were previously associated with statewide landslides.
In the 2000 election the tilt of downstate to R began to show. Gore carried Cook by 750 K but only won the state by 570 K. The difference downstate grows in 2004 as Kerry takes Cook by 840 K but only has a 550 K win statewide.
Obama clearly gets a favorite son effect, but it's instructive to compare 1992 to 2008. The shift of southern IL vs suburban Chicago is quite marked. Since Chicago and suburbs controls an ever larger fraction of the state's vote, the Cook county margins will continue to dominate the state.
The GOP has had a severe erosion in the collar counties as well. The collar counties and Orange County, CA have had kind of the same trajectory, from massively Pub (probably generating along with OC the largest margins in raw votes for the Pubs of any area in the United States), to modestly Pub. The collar counties are what used to float the GOP's boat in Illinois.
