Eight months ahead of the U.S. 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.