From the Politico:
From the fever swamps of the blogosphere to the halls of academia, there is a chorus of voices who have come to the same conclusion about the presidential election: Barack Obama is going to win in November, by something resembling a landslide.
Yet for all the breathless analysis and number-crunching that has convinced observers Obama is en route to an epic victory, there is one key historic fact that is often overlooked — most popular vote landslides were clearly visible by the end of summer. And by that indicator, 2008 doesn’t measure up.
In five of the six post-war landslides (defined as a victory of 10 percentage points or more) the eventual winner was ahead by at least 10 percentage points in the polls at the close of August, according to a Politico analysis of historical Gallup polls. Over the past week, however, Gallup’s daily tracking poll pegs Obama ahead of John McCain by a margin of 2-5 percentage points.
The one exception to the August rule was 1980. Ronald Reagan was trailing slightly in the August polls before surging forward to win by roughly a 10-point margin.