Mr. Romney has carried the wealthier counties. He’s won 52 of them, or 46%. But he has struggled in those less-wealthy counties, winning only 64%– or about 15%.

The vote for the suddenly surging Mr. Santorum looks much more balanced. He has done OK in the wealthier counties, winning 44 of them – or about 39%. But he has also captured 146 of those less-wealthy counties, or about 54%.

The strengths of Rep. Ron Paul and Mr. Gingrich, meanwhile, lie in the less-wealthy counties. Together have only won 17 counties where the median household income was above state median household income and 85 that were below…

Mr. Romney is presumed to be headed to victory in Michigan. He won the state in 2008 and it was his childhood home, when his father was governor in the 1960s. But look at that map above and you see a lot of light green, representing incomes below $40,000. That could be a problem for Mr. Romney.