By the late 1990s, though, Illinois had started to become more Democratic in presidential elections, partly because the Republican's social conservatism in other states had started to alienated many Northeastern and some Midwestern Republican voters. Illinois rapidly became more Democratic in the second half of the 1990s and early 2000s.
The passage describes how Illinois became more Democratic in presidential elections due to the Republican party's conservative social policies in other states, particularly affecting voters in the Northeast and Midwest.