How big will the bounce be?

The bull and the bear case for Harris’s post-convention numbers.

Kamala Harris starts the week as an ever-so-slight favorite in our forecast, but the race is still pretty much a tossup. But Harris’ strong performance at the Democratic National Convention could help her maintain her momentum and increase her lead.

Why? Conventions typically produce short-term bounces in the polls for a party’s presidential candidate. Some pre-DNC polls showed Trump doing better than he has since Harris entered the race, but Harris’ mid-convention and post-convention polls have been strong so far. We’re already seeing the beginning of a bounce in our national polling average — Harris reached a new high of about 49% in our latest update, up from 47% on the day the DNC started. 

But can we count on Harris having a normal-sized convention bounce given how unusual the race has been? And how does the other big news from last week — RFK Jr. dropping out — complicate things?

Take a look at this chart showing the convention bounce for each presidential candidate between 1964 and 2020.¹ On average, candidates’ standing in the polls increased by 5 percentage points after their party’s convention. Most of these gains came from the undecided column or from voters who’d said they planned to vote for third parties: the opponent’s standing typically declines too, but only slightly. 

But as you can see from the table, candidates used to be able to rely on larger bounces. For example, George Bush got a 6-point bounce in 1988 and Jimmy Carter got a 9 -point bounce in 1976. The average bounce between 1964 and 2000 just over 6 points. 

In a highly polarized political environment and a 24/7 news cycle, conventions just matter less now. The average post-2000 convention bounce was only 1.8 points, and the largest bounce in this period was Trump’s 3-point gain in the polls after the 2016 Republican National Convention. Our expectation was that convention bounces this year would be larger than they were during the height of COVID-19 in 2020 — and we rolled back COVID-era adjustments to the model accordingly. But our model still expects a pretty modest bounce – a net gain of only 2 or 2.5 points immediately after the convention. 

If Harris were a typical Democratic candidate, I’d probably just stop here and tell you to expect a 2-point bounce. But Harris’ candidacy is anything but typical. She came into the race late (after Biden’s post-debate downfall), quickly emerged as the only viable replacement nominee, and almost immediately surged in the polls.