Looking at pretty much the final results in Illinois, with Mitt Romney leading Rick Santorum 47%-35%—more than projected by the exit poll, as I suspected earlier this evening—what we see is that Romney won big in metro Chicago, carried the counties with small cities (Rockford, Peoria, Springfield, Champaign-Urbana) Downstate and ran even in the two counties just east of St. Louis, for a solid victory. Romney essentially carried the areas that Barack Obama carried against John McCain in November 2008, and where Abraham Lincoln prevailed over Stephen Douglas in the 1858 popular vote for state legislature which was decisive in their battle for the Senate seat that Douglas had occupied for the 12 years preceding, and which he kept, despite his side’s losing the popular vote, because of the legislative districting plan then in effect.
By the way, it looks like the turnout in Illinois—characterized by many as low—will be higher than in the 2008 Republican primary contest.
