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Gary Johnson averaged just 7 percentage points in 11 polls1 released on Thursday, continuing a string of bad results for the Libertarian Party nominee. At the same time, the number of undecided voters appears to be falling. Those two trends are combining to remove some of the uncertainty in our forecasts — historically, the number of undecided and third-party voters has been strongly correlated with both polling volatility and polling error. The share of voters not supporting the major-party candidates remains higher than it was at this point in the 2012 campaign, for example, but the more it shrinks, the safer Hillary Clinton’s lead becomes.
Clinton and Donald Trump now combine for a little over 84 percent of the vote. That’s the highest their combined share has been since we started issuing our forecasts in June.2 There hasn’t been a huge change, but it’s meaningful.
All this shouldn’t be too surprising. We’re now only about a month away from the election; more voters are making up their minds. In recent elections, moreover, third-party candidates have tended to fade in the polls as Election Day approaches — Johnson and Jill Stein, the Green Party nominee, appear to be following the same trend.
With more voters committed to one of the two major-party nominees, Trump simply has fewer people he can appeal to in order to make up his current deficit, which makes Clinton’s lead more secure. A 5-percentage-point lead with about 15 percent of the electorate undecided or voting for a third-party candidate (about where the race currently stands) is far better than a 5-point lead with over 20 percent of the electorate undecided or voting for a third-party candidate (about where the race was in mid-June). That’s part of the reason that Clinton’s chances of winning the election are in the upper 70s now, while they were in the low 70s back in June.
It’s not yet right to call the 2016 campaign normal, though. While we have seen a decline in the percentage of voters who are undecided or holding out for a third-party candidate, it’s still much higher than it was in the past few election cycles. This is easy to see in a graphic put together by Drew Linzer, who runs the Daily Kos Elections forecast:
Linzer’s chart helps explain a weird quirk of this election. For the most part, Clinton’s lead over Trump has been larger than the one President Obama had over Mitt Romney at comparable points in the campaign. Yet, at those moments, Clinton’s chances of winning have been lower than Obama’s were, according to our forecasts. That’s because of the larger third-party and undecided vote this year, which is why it’s important not only to pay attention to the margin between Trump and Clinton, but also to the percentage that each candidate is getting. Four years ago, Obama was regularly in the upper 40s, while Clinton has generally been stuck in the low-to-mid 40s. If the gap between Clinton and Trump narrowed by a point or two, it wouldn’t hurt her much — as long as she moved closer to getting 50 percent of the vote.
To illustrate this, let’s cut the number of third-party and undecided voters in half, distributing them equally between Clinton and Trump. In that scenario, our model would peg Clinton as a stronger favorite. Instead of winning 78 percent of the time in our polls-only forecast, she would win 85 percent of the time. In our nowcast, which projects the results in a hypothetical election held today, Clinton’s chances jump from 86 percent to 93 percent. And in our polls-plus forecast — our most conservative model — Clinton’s chances go from 75 percent to 80 percent.
The clear uptick in Clinton’s odds in these scenarios is worth keeping in mind as we head into the second debate, on Sunday. Although Clinton would benefit from a wider margin between Trump and herself, she would also benefit if more voters made up their mind — even if those undecideds split evenly between the major-party nominees. The opposite is true for Trump. He would benefit if he could shrink the margin, but he should at the very least hope to make voters think twice before committing to either candidate.