What’s at stake in the first presidential debate?

When Donald Trump and Joe Biden take to the debate stage on Thursday night, it will be a reunion of sorts – although not exactly a friendly one.

A current president has never before debated his predecessor, and the bad blood between these two men will be obvious on the CNN debate stage in Atlanta.

Trump never conceded the 2020 election to President Biden and days after his supporters attacked the US Capitol, he broke with tradition by refusing to attend his opponent’s inauguration.

The two men are now facing off again for the presidency and this debate will mark the first time in this election campaign that millions of Americans are sitting up and paying attention.

The stakes are high and the tension in the room will be a notch above past tussles, as both men try to convince US voters that they deserve their ballot in November.

Thursday’s debate will also mark the first time that Joe Biden and Donald Trump have appeared together since their debates four years ago. The initial encounter in 2020 was an acrimonious affair, punctuated by repeated interruptions and Mr Biden’s frustrated “Will you shut up, man?” demand.

At the start of the second debate that year, delayed by Trump’s Covid diagnosis, the two men did not even shake hands.

This time, both men are out of practice. Neither has participated in any kind of debate in nearly four years, as Trump skipped the Republican primary debates on his way to becoming the party’s presumptive nominee earlier this year.

Incumbent presidents frequently come out flat-footed in their opening re-election debate – a common explanation is that they are rusty or unused to being challenged after four years in the White House bubble. In this case, however, both candidates could face that challenge.

Unlike past debates, this one will be conducted in a cable television studio without a live audience to cheer – or groan. That was a request by the Biden campaign, which reportedly was concerned after a raucous Trump town hall forum hosted by CNN last year.

The debate will also feature muted microphones for candidates during their opponent’s allotted speaking time, which might prevent it from spiralling into the chaos that characterised the first Trump-Biden debate in 2020. But it also could make this version a less memorable affair.

If one listened only to conservative commentators, President Biden will be lucky to make it through the debate without falling asleep, freezing up or wandering around the stage in confusion.

Republicans, from Trump on down, have characterised the president as senile and infirm, a shell of the man he once was.

While these attacks have played upon very real voter concern about the durability of an octogenarian president, it also has set a low bar for Mr Biden’s performance – an expectation that he has exceeded in the past, including during his energetic State of the Union address in early March.

Trump campaign officials recently tried to nudge that bar higher, noting that Mr Biden proved himself to be very effective during the 2012 vice-presidential debate against then-congressman Paul Ryan. They have also questioned the impartiality of debate host CNN.

“Will CNN decide that they are facilitator, or will CNN become a participant?” Trump campaign manager Chris LaCivita asked on Tuesday.

Trump and his campaign have also spread claims that Mr Biden will need to rely on unspecified “performance enhancing drugs” during the debate. The notion has been vehemently denied by President Biden’s team but the seeding of such rumours could lay the ground for post-debate excuses if the president gets the better of his predecessor on Thursday.

Biden campaign spokeswoman Lauren Hitt said the former president was resorting to “lies” because he is “scared of being held accountable for his toxic agenda”.

President Biden may not be the only one with an opportunity to defy expectations, however. Democrats have been warning for more than a year that Trump is obsessed with revenge and retribution, and that he is an aspiring dictator who presents an existential threat to American democracy.

Biden campaign officials have said that Trump “snapped” after his 2020 election defeat and is a different man than Americans elected in 2016.

If the former president can keep his cool for 90 minutes and soften some of his sharper edges, it may help him convince the American public that the dire warnings about a potential second Trump term in office are overblown.

“Biden has got to prove that the perception that he’s too old for the job is not true,” Mike Murphy, a long-time Republican political consultant, told Americast, the BBC’s podcast on US politics.

“Trump’s got to prove that he is not the unlikable madman that half the country thinks he is. So it’s an opportunity for both of them – but also the risk is high.”

Coming into this debate, polling indicated that voters gave Trump better marks on the economy and immigration – two of the top issues for American voters.

Meanwhile, the president was favoured on abortion, healthcare and the environment.

The winner of Thursday night’s debate could well be the candidate who can best land memorable lines on areas of strength while defending his weaknesses.

Can President Biden convince voters that he shares their concerns about a surge in immigration but has been stymied by Republicans in trying to deal with it? Will former president Trump find a way to convince voters another term in office won’t lead to greater restrictions on abortion – particularly given that he appointed three Supreme Court justices who voted to overturn the federal right to the procedure?

Mr Biden has been trying for more than a year to convince the American public that the economy is better than they think it is. He’ll have a chance to make that case again to an audience of tens of millions, but he’ll have to do so in the face of what’s sure to be withering attacks from his opponent, who is expected to focus on the soaring prices and high inflation that Americans have had to live with in recent years.

“We know that Joe Biden is going to try to blame everything on President Trump,” Trump campaign spokesman Jason Miller told reporters on Tuesday.

He added: “Americans know the difference between the Trump economy, which was great, where everyone was doing better, and the Biden economy.”

Last week, the Biden campaign launched a new series of advertisements directly attacking Trump for his recent criminal felony conviction in a New York court.

Recent polls have indicated that the guilty verdict has cost Donald Trump support among the independent voters who could prove decisive in this election.

There’s little question that Trump will be asked about the history-making court case at the debate and that President Biden will be poised to strike. If the former president gets drawn into a tirade against corrupt judges and rigged courts, it could further alienate moderate voters.

“If they can get Trump to burn the mic being a madman on neurotic defence, that that is a very good debate thing,” Mr Murphy said.

Meanwhile, President Biden has a court case of his own to deal with. His son Hunter’s conviction on felony firearm charges was also history-making, and the president – while not implicated in the case – was emotionally invested in its outcome.

Trump may take a few swipes at Hunter Biden, if not about the gun case then about his upcoming tax evasion trial, which could expose controversial details about the younger Biden’s business dealings. Trump will probably attempt to paint Hunter Biden’s conviction as evidence of more widespread corruption in what he calls the “Biden crime family”.

This is the earliest presidential debate in modern US history – held before either candidate has become the formal nominee of their party. That means the showdown could set the mood and the measure of the campaign to come, solidifying some loosely held views about the candidates and better defining the issues and stakes around November’s vote.

But barring a truly catastrophic mistake by one of the candidates, the early timing could mean that when Election Day arrives, this June event will be a distant memory for the majority of Americans who only sporadically follow politics.

Both candidates will have a chance to reset and rebuild from any damage during their tightly scripted national conventions, which are taking place later in the summer. There’s then another debate scheduled in September that could further erase this week from voter memories.

It’s a reality that a senior Biden campaign aide, speaking to the BBC’s media partner CBS, acknowledged.

“The June debate is not a moment that we expect to define the trajectory of the election or move poll numbers in the near-term,” the advisor said, adding that voters will require “consistent time and effort”.

In other words, this debate is the start of a marathon, not the end of the race.