There are four trends to consider when thinking about the general election so far:
As Romney encountered stiffer-than-expected resistance in his march to the nomination, he began to appear vulnerable as a general election challenger. Economic indicators were looking up in the first quarter of 2012, fortifying Obama's position. If that trend had continued into the second quarter, this would not be a close election.
The jobs numbers, released the first Friday of every month, hit the skids in the second quarter. Obama's re-election likelihood tumbled with them. April, May and June were the three lowest job growth months this year.
From early July to the present, we see the influence of Father Time, who is unkind to challengers of either party. By definition, an election is the incumbent's to lose, and his or her opponent has a finite amount of time to successfully make the case for a change in leadership.
Last, we see that the conventions were a net boost for Obama. Romney netted a historically low poll bounce in an average convention and the unfortunate presence of Hurricane Isaac. Obama gained a modest bounce in the polls from his own convention highlighted by President Clinton's policy-oriented address.)