Will the debate between Kamala Harris and Donald Trump matter?

NO PRESIDENTIAL DEBATE has had as much impact on an election as that between Joe Biden and Donald Trump on June 27th. Mr Biden’s performance was so disastrous that his own party pressed him to drop out. Since Kamala Harris, the vice-president, replaced him as the Democrats’ nominee the race has tightened. It is now a toss-up, according to The Economist’s model. On September 10th Ms Harris will face Mr Trump in a second presidential debate. How much difference will this one make?

It was not until 1960, when John Kennedy and Richard Nixon sparred on television, that two presidential candidates debated one another. Since then journalists have ascribed great importance to the events. Candidates are said to gain “momentum”; good performances are “game changers”. Gaffes can dominate the political conversation: debating Jimmy Carter in 1976, President Gerald Ford claimed, ludicrously, that Poland was free of Soviet influence.

Political scientists have been sceptical about how much effect debates have. Robert Erikson of Columbia University and Christopher Wlezien of the University of Texas at Austin analysed presidential elections from 1960 to 2008. They found that polls taken before debates were very close to those taken a week after them. Further analysis by The Economist of data compiled by the same researchers suggests that debates also made little difference in 2012, or even 2016, when the first head-to-head between Hillary Clinton and Mr Trump attracted a record 84m viewers (see chart).

The debates in 2020 stood out—not because of their effect on the polls, but because they were little more than slanging matches. Mr Trump called Mr Biden stupid; Mr Biden dubbed his opponent a “clown” and asked, exasperated, “Will you shut up, man?” One study found it to be the most disrespectful presidential match-up ever.

In normal circumstances, it is unsurprising that debates rarely make a difference. Those who tune in tend to be interested in politics already, and polling suggests that partisans are more likely to watch than independents. Many viewers will have already made up their minds. And in most cycles candidates have been campaigning for months by the time debates come around, either in primaries or from the White House. They are already well known to voters.

This election is different. Ms Harris became the Democratic Party’s champion very late in the race. She is already well-known to those who follow politics closely—not only from her four years as vice-president, but also from her time as a senator and her unsuccessful bid for the Democratic nomination in 2020. But many Americans are still getting to know her. Research suggests that voters gain the most from debates when they know less about the candidates.

So far, voters like what they see: immediately before the debate that caused Mr Biden to withdraw from the race, Ms Harris’s net favourability rating was -17 percentage points; it has since jumped to 1. Americans have already had a taste of her skill as a debater. She took on Mike Pence in the vice-presidential debate in 2020 and came out on top, according to polls at the time. And her career as a prosecutor has given her a steely resolve, which was on display in 2018 when, as a senator, she interrogated Brett Kavanaugh during his vetting for a seat on the Supreme Court. If Ms Harris can show the same skill against Mr Trump, she may win over more Americans during this debate. And in a tight race, that could be crucial. ■