United States Vice President and Democratic Party nominee Kamala Harris will face off against former President and Republican candidate Donald Trump for their first — and potentially only — presidential debate before November’s election. The two have never met before. 

Trump had previously debated President Joe Biden on June 27. Biden subsequently dropped out of the race in July and was replaced by Harris.

The Trump-Harris debate, hosted by ABC News, will take place at 9pm US eastern time on Tuesday (01:00 GMT on Wednesday) at the National Constitution Center in Philadelphia, Pennsylvania.

The latest polling data show the two main candidates in the presidential race locked in a near dead heat both nationally and in a series of swing states expected to determine the outcome of the November 5 election.

Many pundits have suggested Tuesday’s debate could be a defining moment in the campaign as tens of millions of US voters tune in to watch the candidates field questions and trade barbs. But with less than two months to go until election day, could the debate shift voter perceptions of the two candidates?

Here’s what decades of presidential debates, polling and research tell us:

Do presidential debates change election results?

On the whole, research suggests the answer is mostly no.

Harvard Business School Associate Professor Vincent Pons and Assistant Professor Caroline Le Pennec-Caldichoury of the University of California at Berkeley evaluated pre- and postelection surveys in 10 countries, including the US, the United Kingdom, Germany and Canada, from 1952 — the year of the first televised presidential debate in the US — to 2017.

The results showed that televised debates did not significantly impact voter choice.

“There’s this perception that debates are this great democratic tool where voters can find out what candidates stand for and how good they really are,” Pons was quoted in a 2019 article by the Harvard Business School as saying.  “But we find that debates don’t have any effect on any group of voters.”

An analysis published in 2013 by University of Missouri communication Professors Mitchell McKinney and Benjamin Warner considered survey responses by undergraduate students from universities throughout the US from 2000 to 2012.

They too found that general election debates had very little impact on candidate preference with the candidate choice remaining unchanged for 86.3 percent of respondents before and after viewing the debate. 

Watching the debate helped 7 percent of respondents who had not decided who to vote for to make a decision. Only 3.5 percent of respondents switched from one candidate to another.

Still, there have been occasions when debates have boosted the chances of specific candidates. Ask Barack Obama.

The Obama boom

In the 2008 presidential race, Obama was able to achieve a significant lead days after the first debate, which took place on September 26, 2008.

While Obama initially led in the polls, Republican competitor John McCain had caught up, and the two senators were neck and neck from September 9 to 14, according to the Pew Research Center. Obama was at 46 percent, compared with McCain’s 44.

From September 27 to 29, however, Obama surged to 49 percent, and McCain fell to 42 percent.

But what do more recent election cycles tell us about the impact of presidential debates on voter choices?

2020 presidential debates: Almost no change

• Trump and Biden locked horns in two debates before the most recent presidential election, facing off on September 29 and October 22, 2020.
• A poll conducted by New Jersey-based Monmouth University before the first debate showed 87 percent of voters surveyed said the debate was not likely to impact their vote.
• The Monmouth survey proved right. Voting analysis platform FiveThirtyEight’s average of 2020 presidential election polls showed that on September 28, 2020, Biden was at 50.1 percent and Trump was at 43.2 percent. By September 30, Biden was at 50.5 and Trump was at 42.9.
• Similarly, the polling numbers for the two candidates barely changed before and after the second debate.
• Biden won the 2020 election with 51.3 percent of the national popular vote and 306 Electoral College votes.

What the 2016 presidential debates tell us

• Democrat Hillary Rodham Clinton and Trump squared off in three heated debates eight years ago.
• September 26, 2016, was the first debate. The two candidates sparred over everything from the racial divide in the US to Trump’s disparaging comments about a beauty pageant winner. Clinton was on the offensive, Trump defensive.
• Most news reports the day after suggested that Clinton had dominated the debate. But according to FiveThirtyEight’s poll average of 2016, that performance barely moved the needle. Clinton was at 42.4 percent while Trump was at 40.5 percent on September 25. By September 27, Clinton was at 42.5 compared with Trump’s 41 percent.
• By October 8, 2016, the gap between the two had grown: Clinton was at 44.8 percent and Trump was at 39.8. The second debate took place on October 9, but neither that debate nor the third one on October 19 changed polling numbers much.
• On October 18, Clinton was at 45.5 percent and Trump was at 38.9 percent. By October 21, Clinton’s numbers were unchanged while Trump was at 39.1 percent. Opinion polls showed the race tightening marginally in the final days of the election with Clinton still leading comfortably.
• On election day — November 8 — Clinton secured 48 percent of the popular vote compared with Trump’s 46 percent, but Trump won the decisive vote in the Electoral College under the indirect presidential election system in the US.

What about the 2024 debates?

Heading into the debate on June 27, Biden was trailing Trump by a small margin, according to polling averages compiled by FiveThirtyEight.

However, Biden was widely criticised for his performance in the debate. He seemed lost, mumbled and was incoherent at times. From June 27 to July 9, Trump gained about 2 percentage points and was at 42.1 percent support, compared with Biden’s 39.9 percent.

Since Harris became the Democratic candidate, however, the race has changed dramatically.

On July 24, three days after Biden dropped out of the race, Harris was at 44.9 percent support while Trump was at 44. The gap has grown since then. As of Monday, Harris was at 47.2 percent, compared with Trump’s 44.3 percent, according to the FiveThirtyEight average.

Do presidential debates matter?

A large body of research suggests that a key reason presidential debates usually don’t influence voters too much is because most voters who tune in to these televised performances are already committed to a candidate.

However, they can help undecided voters form a preference. And when a candidate is relatively unknown, as was the case with Obama in 2008 or Democrat John F Kennedy in 1960, presidential debates can influence how a candidate is perceived by voters.

In 1960, Kennedy and Republican Richard Nixon took part in four presidential debates. Nixon was the vice president under outgoing President Dwight Eisenhower. A widely held narrative that emerged from those debates suggests that the younger, more energetic Kennedy gained popularity over Nixon among those who watched the debates on television, even though Nixon fared better among voters who listened on the radio. An analysis by researchers at Purdue University in Indiana suggests that one reason for this was that Kennedy “appeared better on television than Nixon”.