Trump will attack Harris' biggest vulnerabilities at the debate. Her response could decide the election.

• The 2024 race is too close to call ahead of Kamala Harris and Donald Trump's first debate.
• Harris enters the debate with clear momentum, but Trump has held strong in battleground states.
• Tuesday night's debate could upend an already chaotic race.

Kamala Harris and Donald Trump will share the stage for the first time Tuesday night, providing voters a look at the two major presidential candidates together after a summer of near-unprecedented upheaval.

There are signs that Harris' rise, which began after President Joe Biden dropped out of the race, has begun to slow. She has dwarfed the former president's fundraising numbers, held raucous rallies, and, most importantly, cut into his polling lead. But a closely watched New York Times-Siena poll released Sunday found that Trump remains a slight favorite.

Polling in most of the battleground states indicates margins that are too close to call. At this point in 2016, Trump was further behind Hillary Clinton. And in both that race and in 2020, pollsters underestimated his support. Trump's also holding strong in Pennsylvania, the most important battleground in the election.

The debate offers both sides a chance to upend the razor-thin race. Harris will face her largest audience yet since securing her historic nomination. Tens of millions of Americans watch these face-offs. More than 51 million Americans tuned into June's debate, the earliest ever, which featured a 2020 rematch that polling had long shown the nation was dreading. In both 2016 and 2020, only the Super Bowl garnered higher ratings.

"So it's like the NFL and the debates by a country mile are the most-viewed things and so all of these people are tuning in, and the research is definitive that voters learn a lot from the debates and they leave the debates confident that they know about the campaign to meaningfully participate in politics," Ben Warner, a professor of political communication at the University of Missouri, said.

Warner said that the timeless Washington parlor game of how much debates truly matter overshadows what he and other researchers have found: that everyday Americans rely on debates to inform their opinions about the candidates.

"You can say, 'Are they really learning the nuances of the policy differences between the candidates?'" Warned said. "I think what's more important is how they feel about the candidates as people, they feel like they know what the candidates stand for, and they are comfortable making an informed decision between the candidates."

Trump will likely confront Harris over her shifting views

This will also be Harris' most unscripted moment so far. Trump and his allies have unsuccessfully tried to goad her into holding more news conferences. They've also pointed out that her website had no policy positions until Sunday evening .

"Kamala has been in a total bubble, she's done half an interview and has otherwise faced no unscripted moments whatsoever," Matt Wolking, who served as the deputy communications director for Trump's 2020 bid, total Business Insider.

Harris and her running mate, Gov. Tim Walz of Minnesota, sat for a joint interview with CNN's Dana Bash , but outside that appearance, the vice president has had few interactions with the press. In comparison, Trump has held multiple news conferences and is trying to appeal to younger men through lengthy interviews with podcast hosts. At the same time, Harris has abandoned many of the most progressive stances she took during her failed 2020 Democratic presidential primary run. Trump's allies hope he calls out what they see as disingenuous flip-flops.

"She's not really quite sure what she believes; that's why she is pretty evasive to answering questions on policy, policy positions that she has supported in the past and supposedly what she does not support now," Wolking said. "I think her embracing two or three of Trump's positions makes her seem like a chameleon."

Trump tore into Harris after she promised not to tax tips , a policy pushed by the powerful Culinary Union in Nevada, after Trump had floated the idea. Harris also hasn't offered much explanation for her changing views. When Bash asked her about it, the vice president repeatedly declared that while some stances may have shifted, "my values have not changed."

A Democratic pollster cautions Harris should be careful in how she responds

Since replacing Biden, Harris' team has embraced a more snarky and trolling tone aimed at getting under Trump's skin. Evan Roth Smith, a Democratic pollster, said that Harris needs to make sure that the lasting moment from the debate isn't a one-liner but rather an imprint of what she would do in office.

"Voters are always, 10 times out of 10, more interested in hearing Kamala Harris talk about what she is going to do and plans to do than anything else, whether it is a zingy rebuttal or an attack on Donald Trump or anything else that could come out of Kamala Harris' mouth," Roth Smith, the lead pollster for Blueprint, a firm backed by the LinkedIn founder Reid Hoffman, told Business Insider.

Roth Smith's polling has found that voters would prefer Harris continue to state her views "in relatively broad terms." He cautioned against getting too bogged down in policy specifics.

"It doesn't matter who is asking you, whether it is the good-faith actors or the bad-faith actors, it would be a mistake for Kamala Harris and the Harris campaign to 60 days out to become a campaign of policy white papers instead of a campaign of priorities, energy, directional focus," Roth Smith. "They seem to understand that and I hope we will see that in the debate."

Harris has been dismissive of Trump's personal attacks herself, but the Democratic strategist Doug Sosnik said he'll be watching closely how the former president handles her. Trump has a long history of attacking female foes in particularly caustic ways, which has underlined and exacerbated his struggle to appeal to women more broadly.

"I think it's particularly relevant in this election, given how Trump treats women in general and how Trump treats Black women in particular," Sosnik said. "To me, that dynamic will be one of the most important to watch."

Sosnik emphasized that viewers might react differently to Trump's actions now than in 2016 when he famously loomed behind the Democratic presidential nominee, Hillary Clinton, during their town hall debate. Candidates are required to stay behind their podiums during Tuesday's debate, but that won't stop Trump's propensity to hurl insults, such as calling Clinton a "nasty woman," an attack Democrats later fashioned into a badge of honor.

"The world is different now than it was eight years ago in a lot of ways, including the behavior he exhibited in 2016. I think he could get away with a lot more then than he can now in terms of offending people, some males, but for sure women voters," Sosnik said.

The race is so close that even a small post-debate bump could be massive

Tuesday's debate will mark the first time Harris and Trump have been in the same room together. It was supposed to be the second debate between Trump and Biden, but the president's disastrous debate performance set off a downward spiral that culminated in his decision to drop out. With perhaps the exception of the Nixon-Kennedy debate, this debate could prove more pivotal to a presidential election than any other in US history.

"I could show you over time that way more times than not, none of those three things matter in terms of the outcome of the election," Sosnik said of the selection of a running mate, the party conventions, and the presidential debates. "In this election, all three probably mattered. The debates, for sure, have mattered and will matter."