Will the debate be decisive?

Less than three months ago, we witnessed a “unicorn” – a debate that changed the outcome of an election.

I'm writing ahead of Tuesday's argument, but here's my advice: don't expect another one.

The June debate helped push Joe Biden out of the presidential race. I say “helped” because it wasn't the debate itself that made the difference. It only changed the race by about half a point. His fellow Democrats widened Biden's deficit by publicly questioning his physical fitness, mental acuity and ability to win re-election.

We find it reassuring to assume that election results are determined by debates.

They give campaigns a focus, journalists a hook for their stories, and voters the illusion that our voting decisions are made on the basis of intellectual debate.

Looking into the soul of a candidate, for example through such tests, sounds wiser than choosing based on vibrations or the choice of our friends and family.

Although it is a practical mythology, the myth has little to do with reality.

It is very unlikely that a debate has ever decided the outcome of a presidential election (with the exception of Biden's defeat, which was not a real election result).

According to one study, the average change in support for candidates caused by the 13 debates between 1988 and 2004 was just one percentage point.

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

Before the first debate of 1960, John F. Kennedy had a lead in the relatively few polls conducted, trailing Richard Nixon by one point. After their debates, Kennedy led by 4 points, although he won by less than one point.

Was JFK's momentum merely halted by the debate (rather than 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 say.

Even what some consider to be the clearest case of an impact is not quite so clear-cut.

In 2000, Al Gore was ahead of George W. Bush before the first debate, which was considered a narrow victory for Gore.

Immediately afterward, polls showed Gore's lead increasing. But within days, his lead shrank as Republicans branded him a “serious exaggerator” and used Gore's comments in the debate as a target, among other things.

Gore's support continued to decline until the third debate, where he emerged 4 points behind Bush. Nevertheless, Gore won the popular vote by about 1 point.

Of course, Gallup's short list of important debates is missing many memorable moments.

At the heart of the 1980 debate was Reagan’s question: “Are you better off than you were four years ago?”

The impact: Reagan's observations showed that he had a five-point lead before and after the debate, which increased when the focus shifted to the American hostages in Iran.

Jimmy Carter's polls came to a different conclusion: They showed that the race was neck and neck even before the debate, and within four days of it, it was neck and neck again.

None of the data suggests that the debate determined the outcome.

Before Michael Dukakis' alleged faux pas – he answered too coolly to a hypothetical question about whether his wife had been raped and murdered – George HW Bush was averaging 54 percent. After the debate, he averaged 55 percent before reaching 53 percent on Election Day. This certainly did not have a decisive influence.

The 2016 debate between Hillary Clinton and Donald Trump is difficult to disentangle, in part because other events, such as the Access Hollywood tape, emerged around the same time, making it impossible to isolate the impact of the debate.

In all three debates, voters chose Clinton by a margin of 5 to 40 points.

But did these debate victories influence the vote?

Before the debates, Clinton was ahead by an average of 2.3 points. After the debates, “Access Hollywood” and a series of other revelations about Trump, Clinton gained a narrow but potentially decisive 2.5 points.

However, Clinton's more modest 2.3-point lead before the debates was closer to her final majority lead (2.1 points) than her post-debate lead.

It can therefore hardly be denied that these debates have had a major impact.

The lesson is simple: we may enjoy talking about debates, but they very rarely lead to decisions.

Mark Mellman is a pollster and chairman of the political consulting firm The Mellman Group. He is also chairman of the Democratic Majority for Israel.

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