32 Years
As David Paul Kuhn of Politico notes, 13 of 14 national polls released yesterday show Barack Obama winning over 50% of the popular vote (the exception being the exceptionally unreliable IBD/TIPP poll).
Lest we forget, it's been thirty-two years since a Democratic presidential candidate won a majority of the popular vote. I'm sure President Jimmy Carter will be happy to see this long streak since his 1976 victory broken.