Will the debate prove decisive? 

Less than three months ago we witnessed a unicorn — a debate that changed an electoral outcome.  

I’m writing before Tuesday’s wrangle, but here’s my advice: Don’t expect another one.   

June’s debate helped force Joe Biden from the presidential race. I say “helped” because it was not the debate itself that did the deed. It only changed the horse race by about half a point. Fellow Democrats increased Biden’s deficit by publicly questioning his physical fitness, his mental acuity and his ability to win reelection.   

We find it comforting to believe that debates determine electoral outcomes. 

They give campaigns focus, journalists a hook around which to wrap stories and voters the illusion that we render electoral verdicts based on trial by intellectual combat.  

Peering into a candidate’s soul, ostensibly through such trials, sounds wiser than voting based on vibes, or who our friends and family have picked. 

Though it’s a convenient mythology, the myth bears little relationship to reality. 

It’s very unlikely that any debate has ever decided the outcome of a presidential election (apart from Biden’s departure, which wasn’t quite an election outcome). 

One study estimated the average change in candidate support caused by the 13 debates between 1988 and 2004 was a mere 1 point. 

Gallup identifies 1960 and 2000 as the only two instances in which debates “may have had a substantive impact on election outcomes,” and even those are, well, debatable.  

Heading into the first debate of 1960, John F. Kennedy had been gaining in the relatively few polls conducted, lagging Richard Nixon by a single point. After their debates, Kennedy led by 4 points in a race he won by less than 1 point.  

Was JFK’s momentum just continuing through the debate (and not caused by it) or did his exchange with Nixon give Kennedy a temporary boost that quickly faded? Or was it all just statistical noise?  

Hard to know.  

What some regard as the clearest case of impact is also not so very clear.  

In 2000, Al Gore led George W. Bush before the first debate, which a narrow plurality scored as a Gore win.  

Polls immediately afterwards showed Gore’s margin increasing. However, within a few days, his lead shrunk as Republicans branded him a “serial exaggerator,” using Gore’s debate statements, among others, as fodder for the attack. 

Gore’s support continued falling through the third debate, from which he emerged 4 points behind Bush. Nevertheless, Gore won the popular vote by about 1 point.  

Of course, Gallup’s brief list of debates that mattered omits many memorable moments. 

The 1980 debate featured Reagan’s “are you better off than you were four years ago” question. 

The impact: Reagan’s tracking showed him maintaining a 5-point margin before and after the debate, which widened as the focus shifted to American hostages in Iran. 

Jimmy Carter’s polls yielded different results, showing the race even before the debate and tied again within four days afterward.  

Neither data set suggests the debate determined the outcome.  

Prior to Michael Dukakis’s alleged gaffe — responding too coldly to a hypothetical question about his wife being raped and murdered — George H. W. Bush averaged 54 percent. After the debate, he averaged 55 percent, before garnering 53 percent on Election Day. Certainly, no decisive impact.  

Hillary Clinton’s 2016 confrontations with Donald Trump are tough to disentangle, in part because other events, like the “Access Hollywood” tape, came to public attention around the same time, rendering it impossible to isolate the debate’s impact. 

Voters gave the debate win to Clinton in all three of their encounters, by margins ranging from 5 to 40 points. 

But did those debate victories affect the vote? 

Going into the debates, Clinton led by an average of 2.3 points. After the debates, “Access Hollywood” and a variety of other Trump revelations, Clinton gained a small, but potentially decisive 2.5 points. 

However, Clinton’s more modest 2.3-point edge going into the debates proved closer to her ultimate popular vote margin (2.1-points) than her post-debate advantage. 

So, it’s hard to argue that these debates had a major impact. 

The lesson’s simple: We may love talking about debates, but they are very rarely decisive. 

Mark Mellman is a pollster and president of The Mellman Group, a political consultancy. He is also president of Democratic Majority for Israel.