In 2008 Obama went up 2+ on Election Night and Romney will probably go up 3+.
Incumbents lose 3 points over the final 3 weeks on Gallup, with most of that coming off the final week. Carter in 1980, Clinton in 1996, Bush in 1992, and Bush in 2004 all lost 3 points in Gallup polling over the final weeks.
If that happens in 2012, Obama will be pushed down to 42% of the vote by election day.