"The strength of the Democratic Party of Cook County is not something that just happened." - Richard J. Daley
The Daleys had many highs and many lows during their 42-year perch above Chicago, but from the electoral crunch of the 1960 election to the landslide of 2008, they were always successful in making Chicagoland one of the most consequential Democratic areas in America.
As the map below shows, during the Daley era, Chicagoland went from being a Democratic, urban, ethnic core surrounded by mainline Protestant Republicans to one that voted uniformly for Democrats. Sean Trende, who produced the map, noted in an RCP article that metro Chicago (outside Cook County) had a Partisan Voting Index of about R+15 in 1972 and one of about D+4 in 2006.
To be sure, many American metropolitan areas trended blue in that timeframe. But Chicago was the training ground for the team of Obama, Axelrod, Emanuel, and Jarrett that won the White House. When was the last time New York, Philadelphia, or Los Angeles could lay claim to the presidency?
At some point during the Daleys' terms, the voters in John Hughes Land (Mark Kirk's constituents) and the mainstream, conservative, unassuming voters on the western suburbs near Dennis Hastert's old district (the onetime base of the Ottenhoff family) started voting Democratic. Ultimately, that may become the Daleys' longest-lasting legacy.