Wall Street Journal has some useful graphs showing the post convention bounce of candidates since 1984. "A review by Gallup of pre- and post-convention polls back to 1964 shows a tendency toward larger convention bounces among Democrats over Republicans and challengers over incumbents, and the candidate whose party holds its convention first also tends to get a slightly larger boost. Interestingly, in four of the last six elections, the winning candidate was the man who had the smaller bounce in post-convention polls -- meaning either he was far enough ahead to ward off his charging opponent, or he came from behind to win in the months between the conventions and Election Day."