• Trump comes in off his best ever debate

• Much improved Harris expected to produce strong show

• Betting likely to spike as global engagement peaks

• Follow our 2024 US Election live blog here

At 2am on Wednesday morning, UK time, the US Presidential Election campaign hits top gear, with Donald Trump and Kamala Harris facing off in their first ever head-to-head debate. Here's what you need to know.

Expect liquidity to spike

There has already been one debate in this cycle, in June, which swiftly led to Joe Biden's departure from the race. The betting drama was unforgettable as Biden imploded live on air, with his odds drifting markedly. While that debate was particularly volatile, it is conventional for liquidity to spike on these occasions. The first debate tends to be the moment at which most non-American bettors seriously engage, and when the head-to-head framing becomes clearer.

Do not be surprised if the market moves sharply in the first few minutes, or if it flips back and forth throughout the night. Going in, Trump has opened up a clear, if not decisive lead in the betting at 1.910/11 compared to 2.186/5 for Harris. it is perfectly plausible that favouritism flips once again.

Which Trump will show up?

Having watched every Trump debate since he entered politics in 2015, I firmly believe he has improved at the art. Being abusive, interrupting, throwing nicknames around, didn't hurt him in the 2016 GOP primary, but it plays terribly in a general election.

Polls taken after the first debate in 2016 saw Hillary Clinton trounce him by around a 2:1 margin. The opener in 2020 against Biden was even worse - a total debacle, prompting horrific reactions from focus groups. However, after he caught Covid a few days later, it was swiftly forgotten.

After recovering, Trump put up a much stronger performance in the second debate. Sure, he lied relentlessly, but his performance was more disciplined. He didn't stray badly off message. That was even more the case in their debate earlier this year. Fact-checkers excoriated Trump, but that was forgotten as the Democrats hit the panic button about Biden's transparent age problems. Trump rather coolly humiliated him.

Harris has improved since 2020

Whereas we know all about Trump on the debate stage, Harris remains a relatively unknown quantity. She badly underperformed when part of a vast field for the Democrat nomination in 2020, coming across as an opportunistic, identity politician. That hung around her for a long time, and fed my doubts when Biden chose her as his running mate.

However, with hindsight, I don't think it is all that relevant. Big field primary debates are a nightmare for all. Trying to be noticed can produce all sorts of errors, and Harris definitely fell prey. But during the later presidential campaign, as VP, and now as nominee, she has improved markedly.

In the 2020 VP debate with Mike Pence, Harris more than held her own. In a recent interview with Dana Bash, she expertly shut down an invitation to rise to Trump's bait over her racial identity with, "Same old tired playbook. Next question." She refused to engage with Trump's obvious attempt to provoke a reaction and confrontation over race.

Trump has underestimated Harris

The Trump campaign seemed to become complacent about Biden being the opponent. Especially after years of relentlessly hammering home his age and cognitive decline paid spectacular dividends on the debate stage.

With Harris, that constant swarm of negative talking points has moved from everybody hates her, to not being black, to being a 'DEI hire', to a lightweight who doesn't do interviews, to most recently, that she has faked a Southern accent. Predictably, Trump says she is dumb and his social media army reeks of deep misogyny.

Democrats will be delighted if Trump tries that sort of tack, alienating voters while she rattles off policies aimed at helping the middle class. Making prescription drugs cheaper, housing more affordable, protecting Medicare, social security and abortion rights. Going for the jugular on Project 2025 - the extreme, anti-democratic policy agenda of Republicans tied closely to Trump, and even more to his disastrous running mate J.D. Vance.

Whatever one thinks of her politics, the idea she is 'dumb' or 'not up to it' is absurd. She is extremely well qualified. Harris was a courtroom prosecutor, and then the Attorney General of America's most populous state. As an interrogator in the Senate, she had very few peers. If Trump is undisciplined, this could be her finest hour.

Flip-flop charges could damage Harris

Equally though, she is there to be attacked on a number of issues. Trump will attack her as being extreme and a flip-flopper, who pushed left-wing policy positions. That she used to support 'Medicare for All' but has since reversed that controversial position is an easy hit. He will accuse her of wanting to 'defund the police', being a communist and failing in her role as Border Czar. (The fact she never held that title won't deter him).

The weak links for Democrats are always the economy, immigration, transgender rights. The last of those will continue to be so, and Trump will go hard on it. If repeating his false claims that schools sanction gender-changing surgery without the consent of parents, it will doubtless be debunked by fact-checkers, yet achieve the purpose of planting that subject into the discourse.

Economy and immigration to dominate

The economy is up for grabs. Harris policies on price-gouging and taxing the rich have popular appeal. She and Tim Walz don't shy away from left-wing rhetoric. Thinking back to 2012, Barack Obama successfully drew a dividing line between his policies for the many, and his opponent Mitt Romney's tax cuts for the rich few. But, regardless of that, she has been in power during a time when Americans have been feeling the pinch. There is plenty of evidence that people feel better about Trump's era than at the time.

Expect Trump to focus relentlessly on immigration on the Southern border. His signature issue. That may backfire. The Biden administration put together a bipartisan bill to deal with this, only to be thwarted by Republicans, who openly said Trump wanted to use it as an electoral issue. I am sure Harris will be well-prepared for this line of attack.

Who wins? Well, the betting in the USA has Harris as red-hot favourite to win according to the polls. Given Trump's record in debates, that seems fair. Debates don't always move the dial but I think this one can, as Harris remains an undefined figure. A strong performance could see her rise significantly in the polls, and impress those non-US bettors who have been keeping their powder dry.

Now read US Election 2024: How to use Betfair odds to predict the electoral college