These surveys have a few things in common. Other than Gravis Marketing, which doesn’t poll the race on a set schedule, they’re all daily or weekly tracking polls. Also, they were all conducted online, via automated calls (“robopolls”) or through some combination thereof; there are no traditional telephone polls in the bunch.
And they seem to tell a consistent story about where the race stands. All of them have Clinton up by about 5 percentage points, give or take a percentage point or two.
That’s not bad for Clinton, but it might seem to suggest that her lead over Trump has abated. A week or so ago, we were seeing leads for Clinton in the mid- to high single digits, with occasional forays into the double digits. Overall, she seemed to be ahead by 7 to 8 percentage points.
So what’s changed? Is Clinton’s convention bounce finally wearing off? Actually, pretty much nothing has changed, according to these polls. Because while we were seeing our fair share of 8- and 10-point leads for Clinton, we generally weren’t seeing them from this group of pollsters, which are (with a couple of exceptions) a Trump-leaning bunch.
Instead, these polls have been steady, at least on average. The table below compares each poll’s most recent result to its previous edition, as well as to each poll’s long-term average (that is, to the average of every previous edition of the survey going back to November 2015). On average, the most recent edition of these surveys shows Clinton up by 4.8 percentage points. But the previous editions showed her up by an average of 4.5 percentage points, so there’s been essentially no change.
||USC Dornsife/LA Times
Recent national polls show a steady race, on average
If you exclude Gravis Marketing, which hadn’t previously polled the race since the Democratic convention, Clinton’s lead has gone from 5.4 percentage points to 4.7 percentage points, also not a meaningful shift.
Comparing each poll’s most recent result to its long-term average is also instructive. Clinton’s current lead of 4.8 percentage points in these polls is more impressive given that they’ve had her ahead by only 2.4 percentage points on average over the course of the year. By comparison, Clinton’s average lead across all national polls dating back to Nov. 2015 has been about 5 percentage points. In other words, these polls have been a Trump-leaning group — they’ve had a pro-Trump house effect of 2 to 3 percentage points. So their showing of Clinton with a lead of about 5 percentage points is consistent with her being ahead by 7 to 8 points overall.
I’m going through this somewhat tedious explanation because it’s basically how FiveThirtyEight’s forecast models calculate their trend line adjustment, which works by comparing polls against previous editions of the same surveys. The idea here is simple. If Trump is gaining ground on Clinton overall, we ought to see him gaining ground in individual polls (including state polls, which are also used to calculate the trend line adjustment). But he isn’t gaining on Clinton, mostly. In polls that had previously surveyed the race since the conventions, Trump is up a point or so in some polls and down a point or so in others — the overall trend is about flat. And in polls that are taking a post-convention snapshot for the first time, Clinton is generally polling better than she did before the conventions. Thus, our polls-only model continues to show Clinton ahead by around 8 percentage points. And it gives her an 89 percent chance of winning, close to where she’s been over the past week.
Still, I’m not quite ready to declare that Clinton’s lead is completely convention bounce free and that we’ve reached a new equilibrium in the race. That’s because I’d like to see another round of high-quality, traditional telephone polls — for instance, the NBC News/Wall Street Journal poll or the ABC News/Washington Post poll. Those polls generally showed excellent results for Clinton just after the conventions, with a larger bounce than some of the tracking polls had. But none of them have surveyed the race twice since the conventions, to give us a sense for whether Clinton’s bounce is holding. This more conservative attitude is closer to the one our polls-plus model takes, which still assumes the race is more likely than not to tighten. It shows mildly brighter prospects for Trump, giving him a 22 percent chance of winning.