With five weeks left to Election Day, President Barack Obama is trying to rekindle some of his 2008 campaign magic on college campuses while also devoting more time to a relatively new format of backyard visits that give him time to explain his policies in cozy, unhurried settings. The two-step strategy, which will play out in four states Tuesday and Wednesday, confronts Democrats? two biggest needs: to pump enthusiasm into young supporters who may stay at home this fall, and to persuade undecided voters that Republican alternatives are unacceptable. On Tuesday night, Obama will headline a rally at the University of Wisconsin in Madison, where he hopes to replicate the raucous, youthful, big-stage events for which he became famous in the 2008 presidential campaign.