Washington, United States - Eight months ahead of the US presidential election, the campaign moves from the parties' nominating contests to the trench warfare of what promises to be one of the longest, most brutal head-to-head showdowns in memory.

US President Joe Biden speaks during a campaign event in Milwaukee, Wisconsin, on March 13, 2024. (Photo by Brendan Smialowski / AFP)

President Joe Biden and his predecessor Donald Trump -- the oldest pair of nominees in history -- emerged this week from a primary season that has inflicted battle scars on both, raising questions over their judgment and mental acuity.

The 81-year-old incumbent Democrat and his Republican foe, 77, see their rematch on November 5 as an existential moment for America and have spent months trading deeply personal insults in a bruising start to the campaign.

Both are unpopular with large sections of a populace wary of handing the keys to the most powerful office in the world to men born closer to the inauguration of Civil War hero Ulysses S. Grant than to November's election.

Trump -- who never left the political stage after defeat in 2020 and faces dozens of pending criminal charges -- is likely to be dividing his time between his signature rallies and court appearances.

"It's clearly a different election this time around -- you could argue both Biden and Trump are weaker, and it's a matter of relative weakness -- but with a long eight months of unknowns in the way," said Joshua Darr, a political analyst and communications professor at Syracuse University in New York.

- 'Age matters' -

The earliest both major parties picked their nominees was in 2000, when George W. Bush and Al Gore were named 243 days before the election. The 2024 campaign is set to be just six days shorter.

Its duration presents unique challenges to strategists looking to manage the candidates' stamina and nervous of election fatigue among voters.

Trump would take Biden's record as the oldest president in history were he to succeed in November and stay the course for a full second term, yet the age issue has been lopsided -- threatening Biden more than his rival.

Trump's gaffes at campaign rallies -- where he regularly confuses names, places and history -- have paled beside a special counsel report that called Biden an "elderly man with a poor memory."

"Age matters. Biden looks older in 2024 compared to 2020, and he is working hard to dispel the notion that he is past his prime," said political historian Mike Cullinane, of Dickinson State University in North Dakota.

The 2024 cycle -- including down-ballot races -- is expected to be the most expensive ever, according to US media citing political ad buys, with total spending likely to exceed $10 billion.

It has been shaped by worries over inflation, surging illegal immigration, anger over the erosion of abortion access and threats to the liberal world order, both domestic and foreign.

Kadir Green, a 25-year-old undecided voter, told AFP in Washington that was "pretty nervous" about the election.

"I feel like the outcome is pretty chaotic. It's very, like, a pressure cooker kind of feeling," said Green, adding the political atmosphere felt "very polarizing."

- Everything to play for -

Trump's aides point to polling showing him ahead in the all-important swing states to argue that he is holding the stronger hand.

Biden says a second Trump term would shake the foundations of democracy as the Republican purges critics, reshapes the federal government in his authoritarian image and pursues vendettas to avenge every perceived slight.

The Democrat's problem is that Trump has been open about these plans, which are already baked into his support among millions of Americans and -- for now, at least -- have done little to harm his poll numbers.

But to win, Trump will need to reach beyond the "Make America Great Again" crowd and convince moderates that he isn't running simply to stay out of jail.

The twice-impeached tycoon has recently been found liable in fraud and sexual abuse lawsuits, and faces 80 felony charges, accused of Espionage Act violations and trying to steal the 2020 election.

"Given that this is a rerun of 2020, there are fewer surprises in terms of the candidates, making context important," Cullinane told AFP. "A major event could change the public perception of the candidates."