Presidential debates matter in American politics. And the one that takes place on Tuesday night between Donald Trump and Kamala Harris – their only currently scheduled face-off – may matter more than most.

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

Even though Kamala Harris has been vice-president for more than three years and a candidate for president for seven weeks, she is still a relative unknown for many Americans. In a recent New York Times survey, 28% of likely November voters said that they needed to know more about the Californian.

That poll showed the race a statistical dead heat – a finding most recent surveys have also indicated, both nationally and in key battleground states. The 2024 presidential campaign has been full of historic tumult, but the American electorate is still sharply – and narrowly - divided.

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

For Ms Harris, the showdown in Philadelphia provides an opportunity for her to flesh out the details before an audience of tens of millions – although she will have to do so while under rhetorical fire from her Republican opponent.

This opportunity is not without risks, however, as Ms Harris could define herself – and her positions – in ways that damage her electoral prospects. She has struggled in the past with answering pointed questions under pressure, and her reluctance to sit for media interviews in the opening weeks of her campaign has denied her the opportunity to hone her linguistic chops.

Although she has tried to present herself as the change candidate in this election, the moderators – and the former president – are likely to press her to defend the Biden administration’s record, particularly on areas where polls show Democrats are weak, such as border security and inflation.

She will also have to explain why she has renounced some of the more liberal policies she embraced during her unsuccessful bid for the 2020 Democratic presidential nomination. She recently has walked back her positions on a fracking ban, decriminalising border crossing and nationalising health insurance, among others.

She has explained these changes as ones made to reflect new circumstances – but they may be viewed by some voters as moves born of political expediency.