Rick Santorum built his presidential candidacy on the premise that he is very different than wealthy, establishment front-runner Mitt Romney, that he is someone who has faced the plights of the average American. Among the hardships Santorum claims to share is an underwater mortgage, the kind that's forcing millions from their homes. "The value of my house is a fraction of what it was when I bought it," the former U.S. senator from Pennsylvania recently told Michigan supporters. But the Republican presidential contender's housing situation is quite unlike the one so many other Americans actually face, according to experts and...