Election Update: Don’t Ignore The Polls — Clinton Leads, But It’s A Close Race

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Two weeks ago, I noted the unusually wide spread in national polls, which showed margins ranging from a 14-percentage-point lead for Hillary Clinton to a 1-point lead for Donald Trump. Now, the results have shifted toward Trump, but the range has narrowed. Of 14 national polls since Oct. 28, when FBI Director James Comey put Clinton’s email server back in the news, all but one show somewhere between a 5-point lead for Clinton and a 1-point lead for Trump.

Here’s the data for all fully post-Comey national polls. For a twist, I’ll sort the polls by their FiveThirtyEight pollster ratings so we can see if the higher-quality polls show a different outcome than the more dubious ones do.¹

On average across the 14 polls, Clinton’s lead is 1.9 percentage points. Chuck out the Los Angeles Times poll, which has been an outlier all year, and her lead is 2.5 percentage points. Or you can take the median instead of the average, which is also 2.5 points. Use only polls rated A-minus or higher? Her lead is 2.6 points. Use live-telephone polls only? It’s 2.3 points. Use only polls used to determine eligibility for the presidential debates? That gets you to 3.5 points. Use only the very recent surveys, which conducted all of their interviews in November? Her lead averages 2.3 points in those.

The point is that however you slice the data, you end up with a Clinton lead in the range of 2 to 3 percentage points. Our national polling average, which weights the higher-rated polls more heavily, is slightly toward the higher end of that range, showing her up by 2.8 points.

For all of the debates about modeling strategies, this is a pretty basic sanity-check. If your model is based on public polls, does it match the consensus of what recent polls are saying? National polls can be a good diagnostic for this, since they’re conducted frequently and often have large sample sizes — the 14 polls I mentioned above, for example, surveyed a combined 23,032 people, which yields a margin of error of only 0.6 percentage points. A national polling average should probably be showing somewhere in the range of Clinton +2 or Clinton +3, or it may be making some strange assumptions.

What about state polls? In order to estimate the national popular vote, FiveThirtyEight’s model combines two different calculations. The first is the technique I described above — a national poll average. The second calculation takes a polling average in each state and then weights the polls based on each state’s share of the national turnout. That estimate also shows Clinton up by 2.8 percentage points. National polls and state polls are telling pretty much the same story, in other words.

How about polls of swing states in particular? Right now, the tipping-point state in our forecast — the state that would provide the decisive 270th electoral vote if the polls got things exactly right — is New Hampshire. There, Clinton leads by only 1.7 percentage points in our adjusted polling average, as several recent polls show Trump tied or slightly ahead, along with others that still give Clinton the lead. Thus, Clinton’s doing a little bit worse in the tipping-point state than she is overall — a sign that she might win the popular vote but lose the Electoral College.

Of course, the polls probably won’t be exactly right. Instead, with so many competitive states, the order could be scrambled (perhaps Clinton will win in Nevada by several points but lose New Hampshire, for instance). So what’s Clinton’s position in the average swing state, weighted based on how likely that state is to be the tipping-point state? She’s ahead by an average of 1.5 points.

You can also see that Clinton’s swing-state advantage is slender based on some of the higher-quality state polls to come across the wire in the past 24 hours:

• Clinton is tied in Florida and down 1 point in Ohio, according to YouGov.
• A Columbus Dispatch poll — conducted entirely by mail! — has Clinton up by 1 point in Ohio instead.
• Clinton trails in Iowa by 7 points, according to the Selzer & Co. poll for the Des Moines Register.
• Clinton is up 4 in Pennsylvania, according to a Muhlenberg College poll.
• And Clinton has only a 5-point lead in New Mexico, according to a poll for the Albuquerque Journal.

That isn’t a great set of results for Clinton. The Iowa and New Mexico polls in particular show a significant underperformance compared with President Obama — and the Iowa poll comes from perhaps the best pollster in the country. The Pennsylvania result is fine for Clinton, and the Ohio numbers aren’t bad — but remember, Ohio is a state Obama won by 3 points four years ago, so we’re really lowering our expectations for Clinton to count polls showing a rough tie there as a good result for her.²

All of this data is nevertheless consistent with Clinton being an Electoral College favorite. She has a 64 percent chance of winning the Electoral College in our polls-only model and 65 percent in polls-plus, putting her somewhere in the range of being a 2-1 favorite.

At the same time, it shouldn’t be hard to see how Clinton could lose. She’s up by about 3 percentage points nationally, and 3-point polling errors happen fairly often, including in the last two federal elections. Obama beat his polls by about 3 points in 2012, whereas Republicans beat their polls by 3 to 4 points in the 2014 midterms. If such an error were to favor Clinton, she could win in a borderline landslide. If the error favored Trump, however, she’d be in a dicey position, because the error is highly correlated across states.

There’s also reason to think a polling error is more likely than usual this year, because of the high number of undecided voters. In national polls, Clinton averages about 45 percent of the vote and Trump 42 percent; by comparison, Obama led Mitt Romney roughly 49-48 in national polls at the end of the 2012 campaign. That contributes significantly to uncertainty, since neither candidate has enough votes yet to have the election in the bag.

To be honest, I’m kind of confused as to why people think it’s heretical for our model to give Trump a 1-in-3 chance — which does make him a fairly significant underdog, after all. There are a lot of ways to build models, and there are lots of factors that a model based on public polling, like ours, doesn’t consider.³ But the public polls — specifically including the highest-quality public polls — show a tight race in which turnout and late-deciding voters will determine the difference between a clear Clinton win, a narrow Clinton win and Trump finding his way to 270 electoral votes.