The interactive graphic included with this article is fascinating. It shows where the candidates drew their support. Most amazingly, Cook County, IL accounts for the entire margin of Obama's popular-vote lead.* The upshot: we Cook Countians really really love Obama.
*Boilerplate explanation: Popular vote totals as arrived at by excluding Florida's and Michigan's unsanctioned primaries and including estimates from the caucuses that didn't release official vote totals.