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 from 39% to 37%. Romney meanwhile has seen his net favorability improve 10 points from +10 (49/39) to +20 (55/35) and his vote share go from 24% to 33%.

What we're seeing in Michigan is a very different story from Florida where Romney surged by effectively destroying his opponent's image- here Romney's gains have more to do with building himself up.

Groups Santorum has double digit leads with include Protestants (up 47-30), union members (up 43-23), Evangelicals (up 51-24), Tea Partiers (up 55-20), 'very conservative' voters (up 54-23), and men (up 40-28).

Romney is leading the field with women (38-34), seniors (42-34), moderates (35-24), 'somewhat conservative' voters (40-34), and Catholics (43-31).

Note Santorum's advantage with union voters. They are one of the keys to a GOP win in Midwestern states. Think of union voters living on an arc that covers the Great Lakes from the UP in Michigan down to IL, IN, to OH, PA, and ending in upstate NY. They are Democrats by tradition, but the GOP can capture them by talking about lunchpail economic issues and jobs. Santorum's more populist economic message is resonating with blue collars - a sign that he won't be the pushover Democrats think he will be if he is the nominee.

Romney's lead among Catholics is surprising given that Santorum shares that faith. But it appears that Romney's support isn't broad enough. He may win Michigan, but unless he can start winning some of those groups Santorum is leading, he won't have much of a chance.