Polling generally becomes more accurate as you get closer to Election Day. The exception to the rule is in the period immediately after the party conventions, when the polls can be a wild ride.
In some past elections, candidates have received as much as a 25-percentage-point lift in the polls after their convention. A bounce of that magnitude is unlikely this year; in fact, there is reason to think that the change in the polls will be quite modest. [cont.]
Nate Silver, New York Times