Obama is three points ahead of Mitt Romney today, and four years ago at the same mid-September point he was eight points ahead of John McCain.
In 2004, John Kerry was slightly behind after the conventions and lost a close race that finally hinged on Ohio and about a 100,000-vote margin for G.W. Bush.
Every historical pattern is meant to be deviated from, and Romney may still alter the polling trend. But early voting starts in about two weeks in Iowa and shortly thereafter in a half dozen toss-up states.