We’ve documented for years how polls tend to rise and fall — in what are often fairly predictable patterns — after events like debates and conventions. In general, what suddenly goes up in polls tends to gradually come back down after a matter of a few weeks. Conventions typically produce polling swings of 4 to 6 percentage points toward the party that just nominated its candidate, for instance — but the polls usually revert back to about where they were before after a few weeks.
We’ve also repeatedly seen this pattern after various Democrats declared for the race this year. Joe Biden, Bernie Sanders, Kamala Harris and even Beto O’Rourke all got noticeable bounces when they officially declared for the presidency, only to fall back to their pre-declaration averages later on.
It looks as though something like this is happening again following the first Democratic debate last month. If you look at the RealClearPolitics average:
- Biden has rebounded to 28.4 percentage points from a low of 26.0 percentage points just after the debate. He was at 32.1 percent before the debate, so he’s regained about two-fifths of what he lost.
- Harris has fallen to 12.2 percentage points from a peak of 15.2 percentage points. She was at 7.0 percent before the debate, so she’s lost about a third of what she’d gained.
Harris is still in better shape than she was before the debates, but she’s currently 16 points behind Biden instead of looking like she’s on the verge of overtaking him.
I’ll be honest … as predictable as this pattern is, it’s easy even for professionals like me to get caught up in the moment, especially in the early stages of a race before we’re using any sort of model to smooth the data out.1 If a candidate rapidly goes from 7 to 15 in the polls, our unconscious, System 1 reflex is to assume the trend will continue, and that the candidate will continue gaining ground — to 20 points, 25 points and beyond. More often than not, though, the candidate loses ground after a sharp rise.
Why this pattern occurs is somewhat beyond the scope of this short article. But one contributing factor may be nonresponse bias — after a good debate for Harris and a poor one for Biden, for instance, Harris supporters may be more likely to respond to polls and Biden ones less so. I tend to think this phenomenon is a little overstated and that an easier answer is simply that a lot of voters don’t have deep convictions about the race until much later, and so bounce around among whichever candidates have gotten favorable press coverage recently. We’ll save that discussion for another time, though.
So it’s worthwhile to be at least a little bit skeptical of rapid, news-driven swings in the polls. By contrast, slow-and-steady gains or losses in the polls — say, Warren’s gradual improvement over the past few months or Sanders’s gradual decline — are often more durable.