WASHINGTON, June 24, 2024 (BSS/AFP) - Joe Biden and Donald Trump square off for a historic US presidential debate this week, with the stage set for what could be a pivotal moment in the 2024 race as millions of potential voters tune in.

The showdown fires the starting gun on what promises to be a bruising summer on the campaign trail, in a deeply polarized and tense United States still convulsed over the chaos and violence that accompanied the 2020 election.

With only two debates this cycle, Thursday's high-stakes clash takes on heightened significance, and both candidates have stepped up their personal attacks, with national polls showing the pair neck and neck.

"The debate is important because it's an opportunity for two well-known candidates to 'reintroduce' themselves to a public that knows them well but hasn't been paying attention," said Donald Nieman, a political analyst and history professor at Binghamton University in New York state.

"The big question is how much of the public -- beyond political aficionados -- will pay attention to such an early debate."

For Trump, the 90-minute clash is a chance to drive home worries about 81-year-old Biden's mental alertness -- although the Republican, 78, has faced age concerns of his own.

For Biden, the first ever debate between a sitting and former president will be an opportunity to underline the legal challenges engulfing Trump and to paint him as unfit for office.

The president will also be desperate to avoid any major gaffes -- which, on this stage, could lose him the November election.

- 'Insult-laden tirades' -

The debate comes in the wake of a criminal trial that has consumed Trump's attention for months -- with his sentencing on 34 convictions for falsifying business records scheduled for July 11.

Both candidates shunned the bipartisan commission that has run debates since 1988, deciding instead to go with CNN for a first showdown unusually early in the year, and another on ABC on September 10.

Abortion, the state of US democracy and foreign conflicts are all issues of concern to voters, although inflation and border security are likely to loom largest.

The last debates between the two men in 2020 were fraught, with Biden at one point snapping "will you shut up, man?" as Trump repeatedly interrupted him.

This time, moderators have more tools than usual to maintain decorum, with the microphones muted except for the candidate whose turn it is to speak.

"Trump is notoriously undisciplined and is likely to chafe at not being able to dominate the event by talking over his opponent and drawing out time with his long-winded, insult-laden tirades," said political scientist Nicholas Creel, of Georgia College and State University.

"Biden is also counting on this debate reminding Americans of the chaos that was the Trump presidency, so Trump being unable to abide by the rules and performing poorly as a result is a very striking possibility."

- More 'presidential'? -

But debates are about soundbites on social media as much as policy arguments, and both candidates will look for explosive viral moments.

"I'll be looking for whether former president Trump tries to become more 'presidential' in any respect, though the campaign trail would suggest the answer to that is no," said Grant Reeher, a political science professor at Syracuse University.

The Biden campaign released an ad last week hitting Trump over his criminal convictions as the president headed to his mountainside retreat at Camp David to fine-tune attack lines and rebuttals.

Trump -- who struggles in granular discussions of policy -- huddled with aides and vice presidential hopefuls at his Mar-a-Lago estate in Florida, where the emphasis was more on tone and broad substance than detail.

Trump would benefit by sticking to a script, highlighting Biden's weaknesses on inflation and immigration and dialing down the bombast, said Nieman, the Binghamton analyst.

"It would disappoint his base, but it would go far to make inroads with suburban, college-educated women," he told AFP.