Why the Biden-Trump debate matters more than you think

The theme of American politics over the last decade is polarization: Specifically, we've seen a lot of it. Some pundits have bemoaned the disappearance of swing voters, for example, and the rule of "negative partisanship" over rationality. As The New York Times asked this February: "Are There Any Persuadable Voters Left?"

Prepare to hear a lot of this discourse again this week, when President Joe Biden and former President Donald Trump face off in the first debate of the presidential general election on Thursday night. I predict much hand-wringing about the effects of this debate — or, to be more precise, how little it could matter. But anyone who thinks debates don't matter in presidential campaigns isn't up on their history.

Campaign events still have small effects

The theory that polarization means debates don't matter is straightforward: When 95 percent of voters vote for the candidate with the same partisan identity as them, gaffes matter less because fewer supporters will be turned off; strong performances, too, matter less because there are fewer voters to court.

Yet while it is true that shifts in public opinion are smaller now than they used to be, persuadable voters do exist and opinions do still change — and in closely divided elections, small changes make big differences. Recent events causing such changes may include then-FBI Director James Comey announcing an investigation into former Secretary of State Hillary Clinton 11 days before the 2016 election (which probably cost her modestly in the polls) and the murder of George Floyd in 2020 (which caused either a durable change in public opinion or a change in who was answering polls — or both).

For 2024, you can add to that list Trump's conviction on 34 felony counts of falsifying business records. According to 538's polling averages, Biden currently has a razor-thin, 0.1-percentage-point lead over Trump in national polls. That margin is so small that it is solidly within the uncertainty interval of our average, but it does represent a real change in the polls over the last few weeks. On June 10, Trump was leading Biden by 1 point in that average. And on May 30, the day Trump was convicted, he had a 1.7-point lead — a 1.8-point difference from today's numbers. Biden's approval rating has also ticked up a few percentage points.

However, Biden's rise in the polls has not translated into a significant gain in our election forecast, which pushes beyond the question of what the polls say today and attempts to answer the question of who is likely to win the election four months from now. Over the last two weeks, when national polls shifted toward the president, his chances of winning the election actually decreased from 53-in-100 to 50-in-100.

That's because there is still a long way to go until Election Day; our model is pricing in about 9 points' worth of change in the national polling margin from now to Election Day, based on historical volatility in the polls. The model also combines the polls with a forecast of the final vote share for each candidate based on economic indicators. That forecast has gotten slightly worse for Biden over the last week, canceling out the gains he has made in the polls.

Presidential debates usually move the polls

The debate on Thursday provides an opportunity for the candidates to interrupt this equilibrium and to send a shock through the system. Again, you may think that polarization means debates don't matter, but my study of 538's historical dataset of presidential polls says otherwise.

Taking all the election polls we have collected since 1976, I used 538's current polling average methodology to calculate historical general election averages for all major party candidates at two points in time: the day a debate was held and two weeks later. Then I calculated how much the polling margin changed as a result of the debate (acknowledging that we cannot completely isolate a debate effect from other stories, news events or dynamics in the polls).

On average, polls have moved by 2.4 points in the two weeks after the first presidential debate of each cycle. That average hides a lot of variance: Sometimes, as in 1988, the candidates are already well known and polls don't change much. Other times, as in 1976 and 2008, one candidate puts on an especially strong performance or capitalizes on existing movement in the race to climb to new heights.

While not a large shift historically, nowadays, in our era of closely contested elections, a 2.4-point change in the race would meaningfully impact each candidate's odds. A 2.4-point gain for Trump would have him clearly pulling away from Biden in Sun Belt battlegrounds such as Arizona and Nevada, putting more pressure on the incumbent to sweep the three northern swing states — Michigan, Pennsylvania and Wisconsin. On the other hand, Biden moving up everywhere by 2.4 points would give him a solid lead in Michigan, Pennsylvania and Wisconsin and put some currently close states like Minnesota, Maine and New Hampshire further out of Republicans' reach.

The predominant pattern, however, is that first debates are usually bad for incumbents. In the seven elections since 1976 with an elected incumbent on the ballot, that incumbent lost ground in the polls in six of them, by an average of 2 points. This would be a significant setback for Biden, who has spent most of the first half of this year consolidating support among Democrats to pull ahead of Trump in the national horse race. But perhaps Biden has little to worry about; much of the reason past challengers have gained after debates is that they made a positive impression on the American people in their first big introduction to the masses. This year, Biden's challenger is a known quantity, so there's less for the public to learn about him.

A small boost for Biden — or Trump — could be the whole ballgame

Both of the candidates face some serious downside risk, however. If you think of potential debate effects as being generated by a statistical distribution (and who doesn't think like that!?), you might say the net effect for each candidate is skewed negatively, potentially with a fat tail — in plain English, there's a higher-than-expected chance of flopping. A long debate risks showing America a portrait of an old, tired president, for example — which they rate among their highest concerns for a second Biden term. On the other side, in the wake of his criminal conviction, Trump has seen the most degradation in support from the least engaged voters — the type you can really only reach with extensive coverage generated by a presidential debate.

Furthermore, it would not even take a big debate effect to change the contours of the race or the winner in November. The 538 presidential forecast currently thinks Biden needs to win the national popular vote by 1.5 points in November to win a majority of Electoral College votes. With his 0.1-point margin in national polls today, a roughly 1-point boost would put Biden very close to where he needs to be to pull off a white-knuckle victory come Nov. 5.

This may all sound incredibly far-fetched to the cynics among you. Can a debate between familiar candidates really change minds? In this hyper-polarized political landscape? The polls have been very stable so far this year, you might retort. But to that I would say: Crazier things have happened, including recently. Races are usually stable until they're not.

The recent rise of political polarization in America is no reason to expect absolutely no effect from the debate Thursday — and even a small bump for either candidate could have a meaningful effect on the race.