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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.

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