Newsweek speaks to political science professor Tom Holbrook, who has spent a lot of time analyzing the data on bumps coming out of party conventions. His theory is that conventions are essentially correcting mechanisms, and the bounce is a reflection of the gap between where a candidate is in the polls, and where they should be. Holbrook sees it as essential that Obama gets a nice bump coming out of the election (in the 6-8 point league), because, “if Obama doesn’t get a big bump out of this convention, I think that will say something about how hard it’s going to be for him to increase his lead in the polls. If he can’t do it substantially over a four-day period when it’s all his show, then I think his campaign should be worried about the months ahead.”
One thing that’s interesting about looking at this chart of post-convention bumps is that it doesn’t necessarily correlate to the outcome of the elections. In fact, out of 11 elections between 1964 and 2004, the candidate with the higher bounce ended up winning only 6 times. Interestingly, check out 1964 — Goldwater got a 12.9% bump, and Johnson got bubkes. Holbrook says that’s because Goldwater was behind by 22 points going in.