Donald Trump-Kamala Harris debate could decide the election

Believe it or not, Vice President Kamala Harris and former President Donald Trump will meet next week in Philadelphia — for the first time.

That’s right — Harris, who entered the U.S. Senate at the same time that Trump entered the White House, who had breakout moments during her cross-examination of Trump’s Supreme Court nominee Brett Kavanaugh and Trump’s two attorneys general, Jeff Sessions and Bill Barr, and who is running against him for president, has never met Trump face-to-face.

Unless they have some kind of backstage run-in before, their handshake at the start of Tuesday night’s ABC News debate will be their first in-person interaction.

For that reason alone the debate will be must-see TV.

But there are other historically atypical reasons why this debate — maybe more than any other since John F. Kennedy vs. Richard Nixon in 1960 — could be determinative in deciding who becomes the next president.

Historians generally agree that presidential debates have mattered little over the years in terms of changing voters’ minds.

That’s because, according to Dustin Carnahan, associate professor in the department of communications at Michigan State University, debate viewers “tend to be among the most politically engaged and thereby likely to have their minds made up well before the debate.”

“For these people,” he continued, “debates serve largely as a spectator sport, watched mainly to see how one’s preferred candidate performs, and with little to no effect on their opinions of the candidates.”

One obvious and recent exception was the debate between Trump and President Joe Biden, which went disastrously for the latter. Even among some Biden diehards, the debate forced a painful reality, which was that he either couldn’t beat Trump or he couldn’t complete a second term. It changed the race completely, but that’s an outlier.

Another reason debates are becoming less important is that fewer people are tuning in. That Biden vs. Trump debate saw a 30% decline in viewership from their first debate in 2020, and viewership has declined steadily from a zenith in 1980, when 80.6 million people tuned in to watch Ronald Reagan take on Jimmy Carter.

Every debate that followed had fewer viewers than in 1980, in some cases 50 million fewer viewers. Until 2016.

The first debate between Hillary Clinton and Trump broke that record with 84 million viewers. After a wild primary season, interest in the Trump/Clinton faceoff was high — how would she handle him? How would he handle her? For similar reasons, that same dynamic is poised to play out next week as well.

Another good reason to be skeptical, however, that this debate will be impactful are the conventions. Typically, candidates will get a postconvention bump in the polls, and they can really matter.

In 1992, Bill Clinton climbed in the polls by as many as 30 points after his Democratic National Convention in New York City. In 2000, Al Gore was behind George W. Bush by as many as 16 points, and by the end of his DNC in Los Angeles, they were tied.

Both Trump and Hillary Clinton got small postconvention bounces in 2016.

Perceptions can sway ‘undecided’ voters

But this year, according to an ABC News/IPSOS poll conducted after the DNC in Chicago, Harris’ postconvention numbers didn’t move at all. And likewise, Trump improved by just a little over one point in the polls.

This likely goes back to Carnahan’s point — that the people tuning into debates and conventions are already politically motivated, and are therefore committed voters whose minds aren’t likely changing.

But this all flips when you’re talking about uncommitted voters.

“[S]ome research has suggested that candidates’ debate performances can impact how favorably they are perceived by voters, which can affect the choices of undecided voters,” says Carnahan.

And undecided voters — especially in the swing states — are largely going to decide the 2024 election.

I spoke with market research expert Elizabeth Jarosz last week, who partnered with IPSOS to poll undecideds about, well, being undecided. She said many of them say they’re waiting for this first debate to decide who they will vote for.

That means Harris and Trump have a huge opportunity Tuesday night to sway the exact voters they’ll need to tip this election in their favor.

The tight margins have been the story of this election, with both campaigns fighting for any gettable new voters. They know this race may come down to a couple thousand votes in a single county in a single swing state.

On Tuesday night, Harris and Trump will square off for the first time, before an audience full of voters who say this was the moment they were waiting for. The big question is, who will make the most of it?

S.E. Cupp is the host of "S.E. Cupp Unfiltered" on CNN.

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