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.