538: How Much Do ‘October Surprises’ Move The Polls?

People are already calling it an “October surprise” — an unexpected moment late in the campaign that could change the trajectory of the election. The news that the FBI is looking into a new batch of Hillary Clinton’s emails (found on the computer of Anthony Weiner) is certainly surprising. And it came in October. But it will take several days to measure its effect on the race, and the real surprise would be a wild swing in the polls. That’s because even the most memorable October surprises of recent history weren’t the game-changers they’re sometimes portrayed to be.

There’s no official list of October surprises, a term that is loosely defined, but I chose six events from past campaigns that would seem to meet the definition, using the benefit of hindsight.

President Lyndon Johnson announces a halt to the bombing of North Vietnam on Oct. 31, 1968, leading to peace talks.
Polling average the week before: Richard Nixon +3.

Polling average in the final week: Nixon +1.

Result: Nixon +1.

Vice President Humphrey couldn’t unify the Democratic base, largely because Johnson, who had escalated the increasingly polarizing Vietnam War, was so unpopular. Johnson’s bombing halt is sometimes referred to as the “first” October surprise, and so you might expect it to have had a clear effect on the polls. The truth, though, is more muddled. Although Nixon had held a large lead through most of October, it was down to just 3 percentage points even before Johnson’s announcement. The polls moved towards Humphrey by another 2 points after the announcement. That’s not nothing, but it wasn’t enough.

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http://fivethirtyeight.com/features/how-much-do-october-surprises-move-the-polls/