I make a big point on this blog of highlighting the importance of the fundamentals in determing the outcomes of presidential elections. Andrew Gelman has a nice graph illustrating how the fundamentals become more relevant as the election approaches (I've added a red arrow illustrating how far out we are from election day right now):
We should expect to see this process continuing in the next few weeks. As UW-Milwaukee's Tom Holbrook points out, one of the key effects of conventions is to bring the polls closer to the expected outcome:
[C]andidates who are running ahead of where they "should" be (based on the expected election outcome) tend to get smaller bumps, and those running behind their expected level of support get larger bumps. In this way, the conventions help bring the public closer to the expected outcome and help to make elections more predictable.
On the other hand, the fundamentals seems pretty closely aligned with current polls so it's not clear how much either candidate will gain from the conventions. Leading models forecast that Obama will receive 51.5-53% of the two-party vote and he's currently at 50.8% in the Pollster.com estimate. At the most, we might expect a small net gain for Obama (i.e. a slightly larger convention bump).