I know that the staunch mobilization of Latinos in California turned that state blue while incoming New Yorkers played a role in ensuring Democratic dominance in Vermont, but why did Illinois go from marginally supporting one party or another by a few points in close elections to consistently voting 10%+ for the Democratic candidate? Was there a demographic development that occurred?
It's all about the generational political conversion of the Chicago suburbs -- western and northern Cook County outside of Chicago, and the "collar counties". Baby-boomer and older middle-class white suburbanites were, overwhelmingly, Nixon and Reagan Democrats. Gen X and Millennial suburbanites lean much more Democrat.
In 1988, George H.W. Bush won 43.4% of the vote in Cook County and and 69.4% of the vote in DuPage County. In 2012, Mitt Romney won 24.6% of the vote in Cook County and 48.6% of the vote in DuPage County (actually losing DuPage County narrowly). Source: uselectionatlas.org.
(This is also a very important part of the political transformation of New Jersey and Connecticut from swing states to safe Democratic states.)