During a pivotal debate in the 2020 US presidential election, one candidate seemed to dominate the stage. They interrupted their rivals at strategic moments, sometimes speaking over them.

They directly confronted an opponent, Joe Biden, generating headlines for days and had critics questioning whether they had breached some sort of unspoken political decorum.

That candidate, however, wasn’t Donald Trump. It was Kamala Harris.

On 10 September, Ms Harris will once again take to the debate stage. But this time, having gone one step further than 2020 by becoming the Democratic candidate for president, she will face Trump in a showdown that poses the toughest challenge of her campaign so far.

Debates have played a major role in Ms Harris’s political career, from her run for California attorney general to her ascent to the vice-presidency. In watching four of her key debates back, it is clear that Ms Harris knows when to seize the spotlight, but also when to stand by as a rival administers a self-inflicted blow.

Ms Harris will be hoping to utilise these instincts against the notoriously combative Trump. Her campaign will also want to dispel longstanding concerns about her political messaging skills that began with her failed run for the White House in 2020, and were only heightened by her fumbling some interviews in recent years.

There is no room for error given how these events are defined by viral clips, so it is just as important for the Harris campaign that she avoids stumbling as it is for her to land a highlight-reel blow.

“She needs to hold her own,” said Aimee Allison, founder of She The People, an organisation that supports women of colour in politics. “And she needs to communicate on the debate stage what she’s fighting for.”

In her earliest debate appearances, Ms Harris found success by letting her opponents dismantle themselves.

In a 2010 debate for the position of California attorney general, moderators asked Ms Harris and her Republican opponent Steve Cooley about a controversial practice known as double-dipping, which allows a public official to draw from their government salary as well as a pension.

“Do you plan to double-dip by taking both a pension and your salary as attorney general?” a moderator asked the candidates.

“Yes, I do,” Mr Cooley replied. “I earned it.”

For a while, Ms Harris said nothing as he defended his position.

“Go for it, Steve,” she eventually retorted. “You earned it!”

Ms Harris’s campaign swifty cut the moment into an advertisement they used to hammer Mr Cooley as out of touch. She won the election by a razor-thin margin.

And during a 2016 debate for a California US Senate seat, Ms Harris’s opponent inexplicably punctuated her closing statement with a dab – a dance move that was popular at the time.

Ms Harris, who looked taken aback, waited a few beats before quipping: “So, there’s a clear difference between the candidates in this race.”

Voters again backed Ms Harris.

Both examples demonstrate Ms Harris’s eye for opportunity on the debate stage, as well as a sense for knowing when it is best to step back. “I think she is someone who uses silence incredibly well,” said Maya Rupert, a Democratic strategist who worked on Julián Castro and Elizabeth Warren’s 2020 presidential campaigns.

As she entered the national stage, Ms Harris proved adept at claiming the floor for herself, even amid a crowded field. One of her tried-and-tested tactics involves openly declaring her intention to speak, compelling her opponents – and the audience – to listen.

The 2020 vice-presidential debate is remembered primarily for one line she directed at Mike Pence as he began to interrupt: “Mr Vice-President, I’m speaking.”

And just weeks ago – illustrating that the riposte was more than a one-off – Ms Harris used the same line on Gaza protesters who interrupted her rally in Detroit. “I’m speaking now,” she told them. “If you want Donald Trump to win, then say that. Otherwise, I’m speaking.”

“She’s using something that a lot of black women have used effectively, which is to insist on their time, and to insist to be heard,” said Ms Allison. “She’s very effective in making sure that she is heard, and respected.”

But perhaps her most memorable debate moment came in 2019, when Ms Harris, then a US senator, stopped all crosstalk during the Democratic primary debate in Miami to challenge Mr Biden over his past position on a policy known as bussing.

She criticised Mr Biden for working with lawmakers who opposed the Civil Rights Era policy of transporting students to schools in different neighbourhoods in an effort to address racial segregation.

“There was a little girl in California who was part of the second class to integrate her public schools, and she was bussed to school every day,” Ms Harris said.

She paused for effect before telling Mr Biden: “And that little girl was me.”

Nina Smith, who was the travelling press secretary for presidential candidate Pete Buttigieg at the time, said the moment made rival campaigns sit up and pay attention.

“What it showed us as a team is if she sees an opening she’s going to go after it,” Ms Smith recalled to the BBC. “I think it made her a rather skilled debater in that regard. It’s definitely something we watched out for, any sort of unexpected punch that could come from Senator Harris at the time.”

“It showed that prosecutorial ability… to really highlight weaknesses in her opponents,” she said.

By the end of the debate, Ms Harris had spoken more than any other candidate except Mr Biden. Her campaign announced it raised $2 million in 24 hours after the debate.

But despite the breakthrough moment and subsequent surge in the polls, Ms Harris later struggled to articulate her own position on bussing. It only served to underscore the concerns with her messaging and ability to articulate a consistent policy position.

The episode was one of many messaging stumbles Ms Harris made that ultimately sank her first presidential bid. Her failure to articulate a consistent policy agenda was one of the most common reasons cited, and it is an issue she needs to clarify quickly at the debate when she will almost certainly be pressed on policy specifics.

Republicans have circulated clips of Ms Harris’ public remarks for years to ridicule her speaking style and cast her as inept. She has used verbose phrases when speaking off the cuff, and while a few turns of phrase have been embraced by her supporters, opponents have often criticised her for a lack of clarity.

In a recent CNN interview, her first since becoming the nominee, she gave an answer on climate change which illustrated the issue. “It is an urgent matter to which we should apply metrics that include holding ourselves to deadlines around time,” Ms Harris said.

On a debate stage, however, speaking time is limited and clarity of message is crucial.

The looming debate on ABC News will be her biggest chance yet to reset public opinion, and past debates show that Ms Harris often brings a sharp toolkit to these events and is able to land blows.

But the pressure of those past debates will pale in comparison to the stakes when she comes face-to-face with Trump for the first time.

Even for the most experienced politicians, Trump presents a formidable challenge, the strategists agreed. In a 2016 debate against his Democratic opponent, Hillary Clinton, he famously stalked her around the stage, drawing all attention to him even when it was her turn to answer.

His first 2020 presidential debate against Mr Biden devolved into an unintelligible melee when Trump kept interrupting. At one point, Mr Biden grew so irritated he snapped: “Will you shut up, man?”

“Donald Trump is a unique and special case in that you never really know what’s coming,” said Ms Smith, who has prepared Democratic candidates for debates. “During debate prep, I would not allow her to get comfortable, in order for her to develop some sort of instinct, or callousness, to anything that could come up.”

Ms Harris, a former prosecutor, is skilled at back-and-forth exchanges on the debate stage. It is something she has also demonstrated during heated Senate hearings when she has grilled Trump officials and Supreme Court nominees.

But the format of the upcoming ABC debate may limit her ability to flex her prosecutorial skills, as the microphones will reportedly be muted when it is the other person’s turn to speak.

This likely means, based on the Biden-Trump debate in June which had the same rules, that she will be fielding tricky questions from moderators as opposed to clashing with Trump.

And when Ms Harris is on the end of prosecutorial questions, as opposed to giving them, she has floundered in the past, such as in a notorious 2021 interview with NBC News’ Lester Holt in which she struggled when pressed on the issue of illegal immigration.

One pitfall that Ms Rupert could envision for the Harris camp is their candidate being drawn into a lengthy debate over facts with Trump. That could muddle the debate for voters, and leave viewers with an impression that he has dominated the conversation.

She suggested a third tactic for Ms Harris to add to her arsenal – not to prosecute, or remain silent, but to ignore.

“She has an important opportunity here to get her point across,” Ms Rupert said, “And not be overly burdened by what he is doing next to her.”