The Trump-Harris debate: Do presidential debates change voter preferences?

United States Vice President and Democratic Party nominee Kamala Harris faced off against former President and Republican candidate Donald Trump on September 10 in their first – and potentially only – presidential debate before November’s election.
Harris and Trump traded barbs on the stage in Philadelphia, Pennsylvania for 90 minutes, on everything from border controls and reproductive rights to the Taliban and bizarre, debunked claims about Haitian immigrants eating cats and dogs. This was the first time they had met.
Many analysts and polls have since crowned Harris the “winner” of the debate, which was held with a backdrop of polling that showed the two main candidates in a near dead heat both nationally and in a series of swing states expected to determine the outcome of the November 5 election.
But as election day fast approaches, a key question remains: do presidential debates really matter and do they shift voter perceptions of the two candidates?
Here’s what decades of debates, polling and research tell us:
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 post-election 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 affect 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 effect 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.
By the time the debates take place, most voters are already aligned with a party, Daron Shaw, professor at the government department at University of Texas in Austin, told Al Jazeera.
On the other hand, “those who are most likely to be affected – independents and undecided voters -are the least likely to watch the debates”, Shaw, an expert on voting and participation, added.
Still, there have been occasions when debates have boosted the chances of specific candidates. Ask Barack Obama.
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?
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.
“The first debate had spectacular effects, essentially providing the stimulus for knocking Biden out of the race. That was a seminal event, and highly unusual”, Shaw said.
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 September 9, Harris was at 47.2 percent, compared with Trump’s 44.3 percent, according to the FiveThirtyEight average.
On September 11, Harris was at 47 percent, compared to Trump’s 44.4 percent.
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.
According to Shaw, debates matter because they show what issues candidates prioritise and hence “they provide cues by which voters construct preferences”.
“The week after the debate is often driven by reaction to issues and narratives from the debate”, added Shaw.
“So debates drive media coverage, which is critical, especially when the Harris-Trump contest is so attenuated”.
Debates 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”.