Biden vs. Trump: 2024 Election Battle Begins

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.

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 has since amassed four criminal indictments -- is likely to be dividing his time between his signature rallies and court appearances.

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 nervousness 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.

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.