The Princeton Election Consortium website run by Sam Wang (a Princeton University neuroscientist who has taken on a second interest in statistical analyses of US elections) has been my go-to site for the last few election cycles because I like his down-to-Earth looks at the state of the polling race, free of hyperventilating over the this or that latest poll. He does not post often but his website contains up-to-date information based on poll aggregation and is well worth keeping track of. If you enjoy statistics and its application in politics, this is the site for you.

His latest post looks at what seems to be driving the recent tightening of the race.

The underlying cause of Trump’s rise appears to be uncommitted Republican voters coming home.
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The narrowing of the race seems to be driven by a break in undecided voters toward Trump. Since early August, Donald Trump has gained about 4.0 percentage points, and is nearing his previous highs for the year. During the same period, undecided voters have decreased by a similar amount.

Until now, undecided voters have been running about 4 points ahead of where they were in 2012. Many of those undecideds were probably disaffected Republicans unwilling to support Trump, a radical candidate unlike any that the U.S. has seen in over 100 years. Trump is not saying a lot relative to his usual inflammatory and divisive statements. It appears that this approach is bringing home the Republican base – and increasing his favorability rating.
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I still expect Clinton’s lead to increase again, on the grounds that she has led all year. Previously, I noted that the national Clinton-vs.-Trump margin in 2016 has averaged 4.5 percentage points. The standard deviation is 2.2 points, comparable to the four Presidential elections from 2004 to 2012. This high level of stability is consistent with the intense voter entrenchment of the last 20 years. Today, conditions seem right for regression to the mean – especially with the first debate coming up on September 26th. I can see the stories now: “Clinton shows renewed vigor” or something like that.

However, the large number of undecideds and minor-party voters provides an additional source of uncertainty. We should wait to see how they shake out; in the last weeks this has benefited Trump. Johnson and Stein support are particularly strong among young voters. Maybe Hillary Clinton should bring out Bernie Sanders or some of the celebrities who showed up at the DNC.

As I said, his site is well worth periodically checking up on.