... Scoring a run in the first inning is worth something, but it won’t shift the win probabilities all that much: there’s too much that can happen later on in the game. We’re now in the political equivalent of the eighth inning, however. A run scored in the eight inning is potentially much more important than one in the first.
The reason I say “potentially” is that it makes a tremendous difference depending what the score is. In a blowout, the eighth inning won’t matter at all. A team down 9-1 is almost certainly going to lose; but so will one that gets a solo home run and trails 9-2 instead.
(The political equivalent: Walter Mondale, in 1984, improved to a 17-point deficit from a 20-point deficit in national polls after his first debate with Ronald Reagan. This may have helped him to carry his home state of Minnesota, and lose the Electoral College 525-13 rather than 535-3.)
But if the score is tied, or if it’s a one-run game, a run scored in the eighth will make a huge difference.
That’s where we find ourselves right now in the presidential race. This election is close and is likely to end up that way. There’s about a 50-50 chance that the election will end up within 2.5 percentage points, according to the forecast, against only a 15 percent chance that either candidate will win by five points or more. ...538
And the October surprise?
... It is now late enough in the race that news events that produce what would ordinarily be a temporary “bounce” in the polls could carry forward to Election Day. ...538
Nate Silver's analyses carry weight because he's been so right so often. In the background of this particular forecast, though, is his experience with baseball. To quote from his Wiki bio:
Silver first gained public recognition for developing PECOTA, a system for forecasting the performance and career development of Major League Baseball players, which he sold to and then managed for Baseball Prospectus from 2003 to 2009.