Vice President Kamala Harris and former President Donald Trump are set to square off for their first presidential debate matchup on September 10 in Philadelphia, Pennsylvania. While presidential debates are already some of the most anticipated campaign events, this debate comes with additional intrigue after President Joe Biden stepped aside from his re-election campaign. In a few weeks, vice presidential nominees Gov. Tim Walz and Sen. J.D. Vance will also debate. Given the finalized two tickets, along with earlier access to ballots among voters in many key states – many are left wondering just how much of an impact the debates will have.

Dustin Carnahan is an associate professor in the Department of Communication at Michigan State University’s College of Communication Arts and Sciences whose expertise focuses on how people engage with political information and how that engagement influences their attitudes, beliefs and behaviors. He answers questions about how much debates matter.

Dustin Carnahan is an associate professor in MSU's College of Communication Arts and Sciences.

What has research shown about the effects of presidential debates?

When it comes to vote choice, research has generally suggested little impact from watching debates. 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, 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. That said, some research has suggested that candidates’ debate performances can impact how favorably they are perceived by voters, which can affect the choices of undecided voters. Additionally, a lopsided debate performance or significant gaffe committed by a candidate can have a lasting effect on candidate evaluations.

However, research into potential indirect effects of debates — by commanding significant media attention in the days prior to and after their occurrence — have offered stronger evidence of their influence. One such effect involves learning, whereby voters make gains in their understanding of the issues and candidates after the debates. But while more knowledgeable voters tend to learn more from viewing the actual debates, less knowledgeable voters have been shown to narrow this gap in the days following debates due to debate-related content being hard to avoid even among those who are the least politically engaged.

Do presidential debates still matter in a time of media abundance?

Despite advances in technology and recent social trends that afford people the ability to ‘tune out’ of the political process, such as the rise of streaming platforms and social media, presidential debates still command significant public attention. According to Nielsen ratings, two of the three most viewed presidential debates in American history occurred in 2016 (Donald Trump and Hillary Clinton with 84.4 million viewers) and in 2020 (Donald Trump and Joe Biden with 73.1 million viewers).

Even insofar as people might have a greater ability to avoid the debates themselves due to increasing media options, they are unlikely to get through the day without coming into contact with some debate-related content. Debates dominate news coverage over a 48-hour period both prior to and following the debate, and content across other media platforms are likely to skew more political than normal. For example, during the aforementioned Trump-Clinton debate in 2016, around 17 million debate-related posts were shared on X (formerly Twitter).

What’s unique about the debates in this campaign cycle?

Of particular note is the late change atop the Democratic ticket, with Harris taking over for Biden in mid-July. This has resulted in a unique scenario. Voters often become quite familiar with candidates across many months or longer, either due to their track record as the incumbent or across several months of campaign events across campaign season. In such situations, debates often provide little new about the candidates – who they are and what they stand for is already fairly well established. In this cycle, Harris is a lesser-known commodity – in terms of specific priorities and policy positions – due to her late entry into the race. This could make this debate more important for the Harris campaign to define her candidacy than in a more traditional year election year.  

There have also been differences in the planning and production of the 2024 debates. Since the late 1980s, the Commission on Presidential Debates, or CPD, has been responsible for organizing presidential debates in the U.S. As a bipartisan group, the CPD would broker agreements between the major-party presidential candidates and their campaigns on matters such as debate format, topic, location and timing. This usually resulted in a series of three debates (and an additional debate between nominees for vice president) beginning in September, with variations in topic focus and format across the three.

This year, the CPD has been left out of the process altogether. The Republican National Committee withdrew from the CPD in 2022, alleging unfair treatment. And in May, the campaigns for then-candidate Biden and Trump  — after some public back and forth — independently negotiated an agreement to hold two debates to take place on June 27 and September 10 (still going forward, though with Harris instead of Biden) without any CPD involvement. Some of the details around the debates also differ from earlier CPD-hosted debates; for example, the campaigns have agreed to mute their candidate’s microphones when it is not their turn to speak and there will be no studio audience for the debate.

How might these changes shape the outcome and perceptions of this year’s debates?

While debates traditionally have small effects, we already have evidence of how 2024 has been different. Biden’s poor performance in June completely upended the race, resulting in his decision to no longer seek reelection. Because of this, the upcoming debate between Harris and Trump could be more consequential for the race moving forward.

While the vast majority of voters already have entrenched views toward Trump, voters are still learning about Harris – what will her leadership style be, and will her presidency largely continue the policies of Biden or diverge in important ways? Research in political science has supported the intuitive notion that persuasive messages have larger effects when trying to change attitudes around lesser-known topics or candidates, suggesting that Harris has more to gain – or lose – from the debate, especially given the very close nature of the race.

Beyond that, there does remain questions about whether any debate-related shifts in opinion are likely to last. Research has shown that any observable effects of debates often decay rather quickly, often overwhelmed by the volume of campaign communication that follows. But earlier access to ballots – voters in key states such as North Carolina, Pennsylvania, and Wisconsin can begin voting via mail within days of the debate, with Pennsylvania voters able to vote early in person as well – means that any small post-debate changes could be consequential in 2024.

Do debates contribute to the spread of misinformation?

The format of debates — where candidates are allotted a specified amount of time to respond to a moderator’s question — does not allow much opportunity for moderators to fact-check candidates’ statements in real time (when, according to some research, fact-checking messages are likely to have a stronger effect in curbing belief in misinformation). Additionally, research has shown that social media discourse during debates can serve to amplify false and misleading claims made throughout the course of the debate, which can result in boosting audiences’ familiarity with — and, potentially, belief in — these statements.

Fact-checkers and news organizations often spend considerable time addressing specific claims made by the candidates in post-debate news coverage. But while fact-checking messages has been shown in research to be fairly effective in correcting misinformed beliefs, audiences for fact-checking stories overlap little with the audience for the debate — especially among more partisan-minded voters less likely to get their news from a source that challenges their preferred candidate.