Presidential debates are important in American politics. And Tuesday night’s debate between Donald Trump and Kamala Harris — their only scheduled showdown — may be more important than most.

Joe Biden’s poor performance during the first presidential debate in late June created a storm of pressure within the Democratic Party, ultimately forcing him to abandon his re-election campaign.

Despite serving as vice president for more than three years and a presidential candidate for seven weeks, Kamala Harris remains a relative unknown to many Americans. In a recent New York Times poll, 28 percent of likely November voters said they needed to know more about the Californian.

That poll showed the race was statistically close — a finding that more recent surveys have echoed, both nationally and in key, hard-fought states. The 2024 presidential campaign has been filled with historic tumult, but the American electorate remains sharply — and narrowly — divided.

That underscores the importance of Tuesday night’s debate, where even small changes in the mood of the electorate can mean the difference between victory and defeat for the candidates.

For Ms. Harris, the showdown in Philadelphia offers her a chance to hammer out the details in front of an audience of tens of millions — though she will have to do so while under rhetorical fire from her Republican opponent.

The opportunity is not without risks, however, as Ms. Harris could define herself—and her positions—in ways that could harm her electoral prospects. She has struggled in the past to answer pointed questions under pressure, and her reluctance to sit for media interviews in the early weeks of her campaign has robbed her of the chance to improve her language skills.

While she has tried to portray herself as the candidate for change in this election, the moderators — and the former president — will likely pressure her to defend the Biden administration’s record, particularly on issues where polls show Democrats weak, such as border security and inflation.

She will also have to explain why she has abandoned some of the more liberal policies she embraced during her failed bid for the 2020 Democratic presidential nomination. She has recently backtracked on her positions on banning fracking, decriminalizing border crossings, nationalizing health insurance, and more.

She has explained these changes as reflecting new circumstances, but some voters may see them as moves made out of political opportunism.

For Trump, the debate offers a chance to regain the initiative in this campaign after a month in which Democrats — with their new nominee and energetic convention — dominated political headlines. He has a history of thriving in the spotlight, setting the terms of political conversations that keep his opponents off balance and keep his issues — particularly on immigration and trade, where his positions have broad popular support — at the forefront of the political conversation.

The former president, however, has his own potential pitfalls on Tuesday. His mixed performance in his June debate with Mr. Biden received little attention because of his opponent’s sometimes catastrophic verbal miscues. Ms. Harris will undoubtedly be a more agile opponent, and his answers will need to be sharper.

At an economic forum in New York last week, he failed to provide a clear explanation of his child care policy. Such verbal ramblings during the debate will provide Democrats with a wealth of campaign fodder.

Trump also needs to be careful when sparring with the vice president — the second female presidential candidate and the first of color. If he comes across as domineering or belittling, he could further damage his already weak support among female voters.

The two candidates used strikingly different methods to prepare for Tuesday night’s event. The vice president — and former prosecutor — was in Pittsburgh, a few hours’ drive east of Philadelphia, to hold mock debates and discuss her policy proposals. The move also allowed her to campaign and capitalize on some local media attention in the largest and most crucial state of Pennsylvania.

Trump — who has participated in a presidential debate in each of the past three elections — has held more informal sessions, including discussing his positions on key issues. Last week, he participated in a town hall forum hosted by the conservative cable network Fox News.

Tuesday promises to be a confrontation of styles and political views.

Although Election Day isn’t until November, early voting will begin this month in several key states, including crucial Pennsylvania.

While this debate could set the political tone for the final two months of the 2024 presidential election, it also marks the last chance for the two candidates to reach voters in states where every vote counts.

North America correspondent Anthony Zurcher provides an inside look at the race for the White House in his weekly US Election Unspun newsletter. UK readers can Register herePeople outside the UK can Register here.