Public Policy Polling has tracked Mitt Romney with a small lead over Rick Santorum in a new Michigan poll. According to PPP, Romney currently commands 39 percent of the vote to 37 percent for Santorum, while Ron Paul is at 13 percent and Newt Gingrich trails with 9 percent. “Compared to a week ago, Romney’s gained six points, while Santorum’s just stayed in place,” the pollsters write. The other difference is a nearly 30-percent drop in Santorum’s favorability over the past two weeks. Once called a “must-win” by Mitt Romney’s campaign and then not a must-win because Santorum opened a huge lead, Michigan is now between a “would like to win” and “sort of must-win.”