It's because the "Collar Counties" (the ring surrounding Cook) are nowhere near as Republican as they used to be, and without the old numbers they used to give Republicans, it's almost impossible to outvote Chicago. Unless the trend in these counties goes back to the Republicans, we can count on Illinois.
And California and New York are not trending Republican.
Although the collar counties contributed to the democratization of Illinois, I think the real gain cam in Cook County. Look at these results of the Dem versus the Repub:
2004: Kerry + 41.1
2000: Gore + 40
1996: Clinton + 40.1
1992: Clinton + 30
1988: Dukakis + 12.4
1984: Mondale + 2.6
1980: Carter + 12.4
1976: Carter + 8.7
1972: McGovern - 7.4
1968: Humphrey + 9.5
1964: Johnson + 26.4
1960: Kennedy +13.1
Carter couldn't win the state in 1976 because he was only at + 8.7, but Kennedy won by 13.1 and won the state.
The same trend occurs in other urban areas of the nation. Take for example Philadelphia county:
2004: Kerry + 61.1 (win)
2000: Gore + 62.1 (win)
1996: Clinton + 61.4 (win)
1992: Clinton + 47.3 (win)
1988: Dukakis + 34.2 (loss)
1984: Mondale + 30.3 (loss)
1980: Carter + 24.7 (loss)
1976: Carter + 34.3 (win)
1972: McGovern - 11.1 (loss)
1968: Humphrey + 31.2 (win)
1964: Johnson + 47.2 (win)
1960: Kennedy +36.2 (win)
Kerry's margin in Philadelphia: 412,000
Kerry's margin in Pennsylvania: 144,000
