Sean Trende has a great look today on why you shouldn't write Mitt Romney off, and includes this historical factoid.
.... no incumbent party has ever held on to the White House while leading by fewer than four points two weeks after its convention; no incumbent president has ever won re-election while leading by fewer than five points (more on the 2004 comparisons later).
In other words, winning under these circumstances would be unprecedented (note also that Ronald Reagan was actually tied with Jimmy Carter in a simple poll average at this point in 1980).
Sometimes these stats remind me of a baseball game where an announcer says "When the Yankees have been leading 4-2 in the 7th inning of games against AL West teams, they have never lost."
The implication being -- the Yankees won't lose in today's particular game under that circumstance.
But while Trende's stat doesn't point, at all, to an Obama loss, it does point to some weakness.