A debate that could change the narrativepublished at 01:38 British Summer Time 11 September

01:38 BST 11 September

Anthony Zurcher
BBC North America correspondent, reporting from Philadelphia

It seems like every day brings the release of a new poll of American voters offering a snapshot of the presidential race, both nationally and in key battleground states. One candidate is up by a percentage or two. One candidate is down by a percentage or two. Sometimes they’re tied.

Taken as a whole, what these surveys tell us is that the race is a toss-up. The American electorate is sharply divided and, with less than two months left until election day, there is no clear favourite.

Kamala Harris and the Democrats have been riding a wave of enthusiasm and positive media coverage since she jumped into the race just over a month ago, but that hasn’t changed the baseline dynamics of this race. Democratic dreams of jumping to a measurable lead over the Republicans were just that – dreams.

Donald Trump’s supporters are sticking with him. The Democrats, while more enthusiastic, are relatively equal in number. The outcome of the race will be decided by the handful of undecideds and low-propensity voters in just a handful of key states who might be coaxed to cast ballots.

For those who have followed American politics in recent presidential elections, it’s a familiar story – and has the makings of yet another tense election night.

With about 30 minutes to go until Trump and Harris face off, let's go over some of the key rules of the debate:

• The ABC-moderated debate is 90 minutes long, with two commercial breaks. You can watch it live at the top of this page
• Trump and Harris can’t talk to their staffers during those ad breaks
• Microphones will be muted for the whole debate, other than when it’s a candidate’s turn to speak
• Props and pre-written notes are not allowed
• But Harris and Trump will be provided a pen and paper, and some water
• There will be no studio audience

• Candidates will have two-minute to answer questions, two-minute rebuttals, and one minute for follow-ups, clarifications, or responses
• A coin flip decided which side of the stage they will stand on and also who will speak last. In this debate, Trump will offer the last two-minute closing statement and Harris will stand stage left

Mike Wendling
Reporting from London

Most Americans won’t be watching tonight’s debate - at least not in its entirety on Tuesday night.

Figures from the Nielsen ratings agency indicated that 51m viewers tuned into the last debate between Joe Biden and Donald Trump.

Their 2020 contests drew a significantly bigger audience - a high of 73m - but that’s still less than half of the number of people who actually voted that year.

For many, the primary way they’ll be exposed to tonight’s battle of words will be through short videos, either on news broadcasts or online.

That’s what makes the battle of the clips - the moments that will be chopped, recycled, posted and memed over the next few days - so essential.

They could be zingers, one-liners that speak directly to a candidate’s weaknesses. Or they could be gaffes or testy exchanges.

The campaigns and their supporters will be watching closely for those potential viral moments, and cutting them into clips and pumping them out to large audiences when they inevitably do occur.

Kayla Epstein

Kamala Harris wouldn’t be standing on stage tonight if Joe Biden had not confirmed Americans’ fears about his age at the last debate in the worst way possible.

The 81-year-old president fumbled through many of his answers, at times sounding incoherent or struggling to finish a sentence.

Now, Democrats have replaced their nominee with a member of a much younger generation.

So the question will be: will Harris bring up the fact that Trump is 78?

It could be effective. Polling consistently suggested that American voters were concerned about both Trump and Biden’s ages, though more were worried about Biden’s ability to do the job.

Now, Democratic voters at least have the younger candidate they craved.

Harris could theoretically question Trump’s fitness for office due to his age.

If Trump wins, he would be the oldest person ever sworn in as president next year.

It’s a double-edged sword, however, as Harris’s own boss is even older than Trump and is still sitting in the Oval Office.

It’s more likely that Harris will make an indirect reference to Trump’s age, with a call for a new generation of American leaders. She might prefer to stick to his record as president when questioning his fitness for office, rather than his age.

Besides, television is a visual medium. The 59-year-old Harris could simply let the contrast speak for itself.

Image source, Getty Images

Kamala Harris - US Vice-President

Age: 59

Party: Democrat

Campaign promises: Harris’s catchphrase is: “We are not going back”. It refers to the policies of former president Trump. She supports women’s right to abortion, launched an economic plan that would ban price gouging at the grocery store, and says she will "end America's housing shortage”.

Major moment of 2024: Her headlining speech at the Democratic National Convention in August acted as an official introduction to the nation as a presidential nominee, barely a month after Joe Biden dropped out of the race.

Donald Trump - former US president

Age: 78

Party: Republican

Campaign promises: Trump says he will “seal the border” to halt illegal border crossings, has pledged across-the-board tax cuts, proposed a 10% tariff on all US imports, and vowed to “drill, baby, drill” to bring down energy prices. Trump has also vowed retribution against his political opponents.

Major moment of 2024: Trump has dominated headlines all year, from surviving an assassination attempt at a political rally, to becoming the first former or sitting president to be convicted of a crime after a jury found him guilty of falsifying business records.

Gary O'Donoghue
Senior North America correspondent, reporting from the debate

Donald Trump's surrogates have been out early in the spin room tonight, saying that all the pressure is on Kamala Harris to perform.

Republican Senator Rick Scott of Florida told me: "The risk is for her. I think the pressure's on her. I think people are gonna watch to see how she does."

He added: "If you don't know who Trump is by now, what are you gonna learn new tonight?"

So should he stay away from the personal stuff, I ask?

"Well, I would talk about the issues... but Donald Trump is Donald Trump."

Bernd Debusmann Jr
Reporting from the debate in Philadelphia

Image source, Bernd Debusmann Jr / BBC News

Among the highest profile surrogates here for the Harris campaign is California Governor Gavin Newsom, who just a few moments ago was addressing journalists in the "spin room".

Newsom said that "this debate matters" for Harris, who must also take the opportunity to "reintroduce herself" to the US public amid a relatively nascent campaign.

"I think the 'why' matters," Newsom told reporters. "What's your motivation? But I think she'll talk about you [voters], not so much herself."

"Beyond that, I think she'll prosecute the case. Not just against Donald Trump and his extremism, but prosecute the case for a future in which we can celebrate our differences."

Additionally, Newsom predicted that the debate will "get interesting", particularly if Harris gets under Trump's skin and angers him.

"He is the most easily manipulated human being I've met," Newsom said. "It's weakness masquerading as strength."

Jonathan Csapo
Reporting from Philadelphia

Image source, Nada Tawfik / BBC

A few blocks away from the convention centre where Trump and Harris will go head to head tonight, a group of pro-Palestinian protesters have gathered outside Philadelphia's City Hall.

From what I can see, there are police lined up near the demonstrators, but everything appears peaceful.

Some colleagues also came across the crowd as they were marching, with some demonstrators holding signs reading "Abandon Harris" - the name of a recently launched group criticizing the vice-president over the US's support of Israel's war in Gaza.

Bernd Debusmann Jr
Reporting from the debate in Philadelphia

Image source, Bernd Debusmann Jr / BBC

While doing the rounds of the spin room, I bumped into Ted Liu, a California Democratic Representative and surrogate for the Harris-Walz campaign.

Liu told me a success - from the Democrats' point of view - will be Harris laying out "a vision for America".

"She's going to do that," he added. "She's going to talk about a new way forward, where she's going to lower housing costs, taking on price gougers, lower food prices and restore reproductive freedoms."

Asked what he expects from former President Trump, Liu retorted that Trump "sounds like the father of lies".

"He spouts an enormous amount of falsehoods," he said, pointing to Trump's border rhetoric compared to falling migrant detentions.

Liu added that, overall, he's confident about the Harris-Walz ticket in the upcoming election.

"The Harris-Walz campaign is the underdog," he said. "But the momentum is on our side."

Bernd Debusmann Jr
Reporting from the debate in Philadelphia

Image source, Cai Pigliucci/BBC News

Another Republican currently making the rounds of the media area of the debate is Doug Bergum, North Dakota Governor and one-time challenger for the Republican nomination.

Speaking to reporters earlier, Bergum said he "wants Trump to win" and is confident of victory.

Bergum added that "when" Trump wins, he'll have a "strong" list of people who want to take part in his administration.

Just a short while before speaking to reporters, he was at the nearby Reading Market, where he was seen perusing honey options at one of the food stalls.

He was asked about this in the spin room.

"North Dakota is the number one honey producer in America," he said. "Number two could double their production and we'd still be number one. I just had a chance to educate people."

Mike Wendling
Reporting from London

Misleading videos, disinformation and straight-up fake news stories are unfortunate and persistent aspects of modern political contests.

But it’s less likely that large numbers of people will be tricked by manipulated footage of tonight’s debate.

The reason for this is simple. With so many eyes on a live, well-hyped broadcast event, any manipulation of debate footage is pretty easily found out.

The fake stories that go really viral, might have some relation to real-world events, but they usually start life as social media rumours.

Take for instance the claim that the Harris campaign used artificial intelligence to manipulate the pictures of large crowds greeting her at a rally, or the persistent rumour that the assassination attempt against Trump was somehow “staged”.

These false stories touched on public events, but grew out of online chatter on the fringes before making an impression on millions of more mainstream news consumers.

North America correspondent Anthony Zurcher makes sense of the race for the White House in his weekly US Election Unspun newsletter.

Readers in the UK can sign up here. Those outside the UK can sign up here.

Sam Cabral
Reporting from Washington

Image source, ABC News/ Al Drago

Tonight's event is Donald Trump's seventh general election presidential debate since he first ran in 2016.

By contrast, it will be the first for Kamala Harris.

When Harris ran unsuccessfully in the 2020 Democratic primary, she participated in five debates, notably landing a blow on future boss Joe Biden at the first event - but fading from view in later showdowns.

Not only that, Harris and Trump have never met in person or even directly spoken to each other.

In fact, the only time they've ever even shared the same room was when Harris, as the junior US senator from California, watched Trump's State of the Union addresses from the Capitol Hill gallery.

That all changes in Philadelphia, where - inside the National Constitution Center museum - they will stand mere feet apart over the course of 90 minutes.

There are 50 states in the US - most of them nearly always vote for the same party. The few where both candidates stand a chance of winning are known as battleground states.

Right now, the polls are very tight in the seven states considered battlegrounds in this election, which makes it hard to know who is really garnering the most support right now.

The latest averages compiled by polling analysts at 538 suggest that less than one percentage point separates Kamala Harris and Donald Trump in five of the states - Arizona, Georgia, Nevada, North Carolina and Pennsylvania. Harris is currently up by two points in Michigan and three in Wisconsin.

But every poll has a margin of error that means the numbers could be higher or lower in reality, so these states are too close to call for either candidate at the moment.

We’ll be tracking the numbers in the battlegrounds every day as we approach the election.

Ottilie Mitchell
US reporter

Bernd Debusmann Jr
Reporting from the debate in Philadelphia

Image source, Bernd Debusmann Jr/BBC News

Just a short while ago, I arrived in the "spin room" of the media zone near the debate site, where campaign officials and surrogates will be doing the rounds with reporters throughout the afternoon and evening.

The first people I saw entering the area were Anthony Scaramucci, Donald Trump's short-lived White House Communications Director, and Olivia Troye, a Trump-era national security official who has become a vocal critic of the former president.

Both are here to express their support for Kamala Harris.

Speaking to reporters, Troye pointed out that "many people" who worked for Trump "have said they don't support him for the presidency".

"I think that speaks volumes about where the Republican Party is today," she adds. "[Trump] is a contradiction to the values of the Republican Party, a contradiction to the values of the party of Ronald Reagan."

Troye said that she remains a conservative - saying "my values have not changed" - and disagrees with some of the Harris-Walz campaign's policy points.

Nevertheless, she said she sees Harris and Walz "representing those values" better than Trump.

Regan Morris
Reporting from Pennsylvania

Image source, EPA

In Pennsylvania, one issue keeps coming back up for Harris: fracking.

It’s a big part of the economy in parts of the state, which was once considered part of a “blue wall” of Democratic support.

“It wasn't until oil and gas really had its rebirth here in the area that there were jobs to come home to,” said Jen McIntyre, a local who now works in the industry and intends to vote Republican.

The Democrats' advantage has been shrinking in Pennsylvania since 2008. President Biden won it by a hair in 2020, and it could go either way this year.

And that’s put Kamala Harris in a tight spot.

In 2019, when she was gearing up to run in the Democratic primary, Harris said she wanted to ban it. But she’s since changed her position.

Donald Trump – who has said his policy is “drill, baby, drill!” – won’t let people forget her past comments.

“She’s against fracking, she’s against oil drilling, she wants everybody to have one electric car and share it with the neighbours,” Trump told rallygoers in Pennsylvania recently.

Who voters will believe remains to be seen.

Tom Bateman
Reporting from Pennsylvania

Image source, Tom Bateman / BBC

In the run up to the debate, we’ve been on the road meeting voters in rural western Pennsylvania - the other side of the state from the host city of Philadelphia. This state is a critical battleground as the opinion polling suggests voters are split in their support for Harris and Trump. So in an election fought in the tightest of margins, the candidates’ performances tonight have the potential to shift things.

Out here in the countryside, where Trump dominates, Harris supporters have been getting out into the neighbourhoods. They are trying to persuade Democratic voters to stay engaged with the campaign and to not stay at home on polling day.

“It's giving visibility to people who feel like they are lost in a sea of Trump signs,” says Bridgette Jackson, a local gas station worker who’s taking a day off to canvass in Altoona, Blair County.

The mum of two says the issue most people on the doorstep are concerned about is women’s rights.

She’ll be watching the debate closely. Bridgette suggests voters will make judgements based on more than just words: “I’m really hoping to see some solid policy plans,” she says.