The problem for Santorum last night, according to the exit polls, is that he didn’t break through in two key areas. Asked what is the most important issue for them, 55 percent of Michigan voters said the economy. Romney won them by 17 points. Asked what was the most important candidate quality, a plurality of 32 percent said the ability to beat President Obama. Romney trounced Santorum 61 percent to 24 percent among these voters. Another 22 said having the right experience was most important and Romney clobbered Santorum among them, too. In contrast, Santorum soundly beat Romney among those voters (24 percent) who wanted a candidate with strong moral character and (16 percent) who wanted a true conservative. But there weren’t quite enough of them. There is a wide practical streak among regular Republican voters and as long as Santorum can’t effectively tap into it his candidacy is going to be limited in its potential.