Washington DC - Republican vice presidential candidate JD Vance described the last-minute entry of Kamala Harris into the US presidential race as a "sucker punch," according to a recording of his remarks at a fundraiser obtained by The Washington Post.

In remarks contrasting with his campaign's public messaging – but revealing the upheaval in a race that had seemed set to feature Donald Trump against Joe Biden – Vance told donors over the weekend that "all of us were hit with a little bit of a political sucker punch."

The Vance-Trump campaign has insisted that the Democratic vice president's entry into the race won't upend their strategy and that Harris is tied to her boss Biden's policies.

"Nothing fundamentally changes," senior Trump advisor Jason Miller said at the Republican National Convention earlier this month, as speculation was brewing that Biden might drop out.

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"Whether it's Joe Biden or Kamala Harris, or any other radical liberal Democrats, they all share responsibility for the failure of destroying our economy and ruining our borders."

But Vance reportedly warned that "Harris does not have the same baggage as Joe Biden, because whatever we might have to say, Kamala is a lot younger."

"And Kamala Harris is obviously not struggling in the same ways that Joe Biden did."

Harris has emerged as the all-but-sure Democratic nominee after 81-year-old Biden's shock decision to step away from his reelection bid, dogged by concerns over his age and mental acuity.

The decision threw the US presidential race into uncharted waters, as Harris, Biden's vice president, sprints toward the November 5 vote on a drastically shortened timeline.

But her entry has also sparked enthusiasm among a vast swath of voters who had soured on a Biden-Trump rematch – and it cements 78-year-old Trump, prone to his own rambling and confusing speeches, as the oldest ever US presidential nominee.