What was once a 15 point spread has closed to 4 points. PPP reports: The Republican race for President in Michigan has tightened considerably over the last week, with what was a 15 point lead for Rick Santorum down to 4. He leads with 37% to 33% for Mitt Romney, 15% for Ron Paul, and 10% for Newt Gingrich.The tightening over the last week is much more a function of Romney gaining than Santorum falling. Santorum's favorability spread of 67/23 has seen no change since our last poll, and his share of the vote has dropped only 2 points...