By Nate Silver – – – – – –
The race between Hillary Clinton and Donald Trump has tightened. Clinton, whose lead over Trump exceeded 8 percentage points at her peak following the Democratic convention, is ahead by 4 or 5 percentage points today, according to our polls-only forecast.
The tighter margins in the polls, which reflect a loss of support for Clinton along with a modest improvement for Trump, have come gradually over the past few weeks. The evidence of a tightening has become more widespread, however, and it’s particularly clear in polls that surveyed the race just after the conventions and are retaking its temperature now. Fox News’s national poll, for instance, had Clinton up by 9 points just after the conventions (in the version of the poll that included third-party candidates) and has her up by 2 points now.
There isn’t any guarantee that Trump will continue to gain ground. Over the course of the year, polls have oscillated between showing a dead heat at Trump’s best moments and a lead of 8 to 10 percentage points for Clinton at her peaks. We’re about halfway between those goal posts now. It’s plausible that the recent shift reflects Clinton’s convention bounce wearing off — reversion to the mean — as much as it does momentum for Trump per se. Most importantly, Clinton is still ahead, with a 74 percent chance of winning according to the polls-only model and a 70 percent chance according topolls-plus.
But what if the race continues to tighten? I’ve often heard Democrats express a belief that Clinton’s position in the swing states will protect her in the Electoral College even if the race draws to a dead heat overall. But this is potentially mistaken. Although it’s plausible that Clinton’s superior field operation will eventually pay dividends, so far her swing state results have ebbed and flowed with her national numbers.
Take Wisconsin, for example. At her peak, Clinton had a double-digit lead there, according to our polls-only forecast. By Wednesday morning, it had declined to an estimated 7 points, as a result of our model’s trend line adjustment — which adjusts polls in all states based on shifts it detects in the race overall — along with data from the Ipsos 50-state tracking poll.
We know that some readers don’t like the trend line adjustment. But if anything, the model hadn’t been aggressive enough. Two highly rated, traditional telephone polls of Wisconsin came out Wednesday, and they showed Clinton up by just 3 points and 5 points. The 3-point lead was in apoll from Marquette University, which had Clinton up by 13 points just after the Democratic National Convention. (As of Thursday morning, Clinton is projected to win by 5 points in Wisconsin, according to the polls-only model.)
Usually, the trend line adjustment helps the model peg what forthcoming polls will look like in a state even if there haven’t been many of them recently. When Clinton established a roughly 8-point lead nationally in August, for example, it figured we’d see polls showing her with leads of 10 to 12 percentage points in some of her better swing states, such as Michigan and Colorado, along with leads of 5 to 6 percentage points in swing states that are just slightly redder than the country as a whole, such as Ohio and Florida. And that’s pretty much what we saw, at least on average. Now that the race has tightened to 4 or 5 points nationally, the model expects to see narrower leads — along with some polls showing a tie or Trump slightly ahead in the more red-leaning swing states:
||ADJUSTED POLLING AVERAGE
Clinton’s projected leads have shrunk in swing states
Keep in mind that these numbers are self-correcting. For instance, the model expects new Ohio polls to show Clinton ahead by 2 or 3 percentage points, provided they don’t have a strong Trump-leaning or Clinton-leaning house effect. If new surveys deviate significantly from that range, the model will adjust itself accordingly. But usually this method gets things about right. Swing states are swing states for a reason — they closely follow the overall national trend.
The other thing to notice about Clinton’s swing state polls is that they aren’t especially strong (or weak) relative to her national polls. At her post-convention peak, Clinton’s path of least resistance to 270 electoral votes appeared to run through a set of states that included Pennsylvania, Wisconsin and New Hampshire, among others. But in Pennsylvania, the most recent polls have Clinton ahead by margins ranging from 3 to 8 percentage points — perfectly fine, but not that different from her national numbers. We haven’t gotten much data recently from New Hampshire, but it can be swingy, and the most recent numbers from the Ipsos poll (caveat: very small sample size) showed Trump ahead. We did get some high-quality polls from Wisconsin, and, as I mentioned, they weren’t that good for Clinton.
Overall, Clinton’s leads in the tipping-point states — the ones most likely to determine the Electoral College winner in a close election — average about 4 percentage points, close to her numbers in national polls.
With a tighter race, the model’s expectations for Clinton are lower. A new poll showing her up by 5 or 6 points in Florida or Ohio — which would have been a ho-hum result a few weeks ago — could be a terrific one for her today, depending on the pollster.