Trump-Harris face-off: Do presidential debates change voter preferences?

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?

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.

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.