Tuesday's presidential debate presents rare opportunity to shape election: Experts

How much difference do presidential debates make?

In the past, they've rarely influenced an election's outcome, according to historians who spoke with ABC News.

But the June debate between President Joe Biden and former President Donald Trump was a rare exception, they say, ultimately leading to an unprecedented change in the Democratic presidential ticket.

Now as Vice President Kamala Harris prepares to take on Trump in their first presidential debate on Tuesday, experts predict the matchup could potentially produce a similar consequential and history-making moment that could sway undecided voters -- a key voting bloc that could determine who wins the November election.

"Given we've never been in this circumstance … and that is going to generate a lot of interest," Aaron Kall, director of debate for the University of Michigan's Debate Program, told ABC News. "And with that increased interest and audience, there is more opportunity to appeal to voters who are still on the fence."

Experts who have analyzed previous debates said this one will be pivotal for many voters since it follows Harris' entering the race as well the naming of the campaigns' two running mates, Minnesota Gov. Tim Walz, and Ohio Sen. JD Vance.

The ABC News presidential debate will take place on Sept. 10 at 9 p.m. ET and air on ABC and stream on ABC News Live, Disney+ and Hulu.

Kathleen Hall Jamieson, director of the Annenberg Public Policy Center at the University of Pennsylvania, who has researched presidential debates, told ABC News that the majority of debate watchers usually are highly informed viewers who have already decided which candidate to back.

In this case, though, Harris is coming as a relative blank-slate, Jamieson said.

"Because she is not as well known as Trump is there is more capacity for the debate to change her status as an unknown," she said.

Although Harris immediately hit the ground running since Biden bowed out in mid-July and then got the news media spotlight at the Democratic National Convention, she will have the opportunity to draw contrasts with Trump -- and in some instances Biden -- during her matchup, according to Kall.

"This is the optimal opportunity to introduce herself," he said.

The debate will mark the first time that Trump and Harris will be seen face-to-face and viewers will be able to see how both react to the jabs likely to fly during the 90-minute showdown.

Kall noted that campaigns have made those memorable one-liners a critical part of their debate preparation, especially with the rise of social media, in order to give their candidates' message a longer life in the fast-changing news cycle.

Jamieson noted that this tactic was key in the June debate as the Trump campaign, the press and others kept repeating the footage of Biden's faltering debate performance that led to calls for him to drop out.

"If you looked at the polls after the June debate, Biden didn't really lose any support, but that messaging was enough to turn the tide," she noted.

Jamieson said the campaigns will likely be laser-focused on making sure their candidate secures victory as there is no immediate indication that they will have another televised matchup before Election Day.

In addition, the debate will take place weeks just as some states begin their early voting procedures.

"The proximity of the debate to your ballot matters," Jamieson said. " The ability to cast your ballot when it is close to the communication of the debate increases the possibility that the debate will shape your vote."

Kall noted that the debate date in June did give the candidates more time to recover any gaffes, however, given the late change to the ticket and the start of early voting, a candidate may not have that luxury.

"We've seen in the past, including the 2020 debates, that a candidate can carry the strengths of a good debate from October to the election," he said. "The same can happen now."