Jim Antle observes:

Rick Santorum is supposed to be the Rust Belt candidate. Perhaps if he stuck to his economic populist message he could have been, though that might have put a ceiling on his conservative support elsewhere. He has clearly lost that mojo.

It’s possible that Santorum hasn’t been well-suited to appealing to Rust Belt states for a very long time. His “economic populist message” was not a very strong one. His recent record on trade and the auto industry made him indistinguishable from Romney. A lot of his working-class appeal is the result of a negative reaction to Romney rather than a positive embrace of Santorum.

Santorum’s showing in Illinois was less impressive than many expected, but this is not hard to explain. Just days before an election in a state where 90% of the primary’s voters resided in urban and suburban areas, Santorum made a point of identifying Romney as their natural candidate. Romney was the candidate who won “in and around cities,” and Santorum thought this was proof that there was something wrong with Romney. Urban and suburban voters probably didn’t need Santorum’s guidance, but they voted accordingly. Romney won the collar counties by 21 points and the Cook County suburbs by 30. Statewide, he won the urban vote by 19 points and the suburban vote by 14.