US Vice President Kamala Harris’s campaign has trolled Donald Trump, calling him “old and weird” after a Fox News interview.

The campaign released a “statement on a 78-year-old criminal’s Fox News appearance”, roasting Trump for his age and honesty after the Thursday interview that followed President Joe Biden’s Oval Office speech.

“After watching Fox News this morning we only have one question, is Donald Trump OK?” the campaign said, adding that Americans must be thinking “Trump is old and quite weird” and that “this guy shouldn’t be president ever again”.

It follow Trump describing Biden’s speech as “terrible” and his exit from the presidential race as a coup.

“I think it was a coup. They didn’t want him running. He was way down in the polls, and they thought he was going to lose,” Trump said.

“They went to him and they said, you can’t win the race, which I think is true, unless I did something very foolish, which I wasn’t going to do. And I think he was so far down and they said, ‘You’re not going to win, and you’re not in great shape, and you did poorly in the debate’. I think the debate started everything.”

The campaign’s statement was released a few minutes after Trump’s interview.

The Harris campaign’s response landed in inboxes only minutes after Trump’s interview. It also called out Trump for “lying and making threats” and praising abortion bans while criticising mail-in-voting.

“If anyone wants an alternative, Kamala Harris is offering one,” it said.

The speed of the biting response is likely to appeal to younger voters – and came as Harris quickly garnered more than 300,000 followers after joining social media platform TikTok on Thursday (local time).

It also came as the latest polls show her narrowing the gap with Republican rival Trump as her swift emergence as the successor to 81-year-old Biden as the Democratic presidential candidate in the November 5 election shakes upa stagnant presidential race.

In a speech to the American Federation of Teachers in Houston, Harris, 59, focused on economic policy and workers’ rights, touting plans for affordable healthcare and child care and criticising Republicans for blocking gun limits in the wake of school shootings.

“Ours is a fight for the future,” Harris told a crowd of about 3500.

“We are in a fight for our most fundamental freedoms. And to this room of leaders, I say: Bring it on.”

A series of polls conducted since Biden ended his re-election bid on Sunday, including one by Reuters/Ipsos, show Harris and Trump beginning their head-to-head contest on roughly equal footing, setting the stage for a close-fought campaign over the next 3½ months.

A New York Times/Siena College national poll published on Thursday (local time) found Harris has narrowed what had been a sizeable Trump lead.

Trump was ahead of Harris 48 per cent to 46 among registered voters, compared with a 49 per cent to 41 in early July, following Biden’s disastrous debate performance that led to a wave of Democratic calls for him to step aside as candidate.

While nationwide surveys give important signals of American support for political candidates, a handful of competitive states typically tilt the balance in the US Electoral College, which ultimately decides who wins a presidential election.

Harris also got good news on that front as Emerson College/The Hill published a poll finding that she had begun to close the gap with Trump in five critical battleground states: Arizona, Georgia, Michigan, Pennsylvania and Wisconsin. Trump still narrowly leads Harris in all but Wisconsin, which is tied, according to the poll of registered voters in those states.

Together, the polls suggest that while Trump retains a narrow advantage, he has not seen the sort of bump in support following last week’s Republican National Convention that candidates hope to get out of the highly scripted, televised and expensive events.

Harris still has the Democratic National Convention to come in August.

On Wednesday night, Trump laid into Harris in his first rally since she replaced Biden atop the ticket. He continued his criticism online on Thursday.

“We’re not ready for a Marxist President, and Lyin’ Kamala Harris is a RADICAL LEFT MARXIST, AND WORSE!” he posted on his social media platform.

The Harris campaign released its first video advertisement online on Thursday. Harris narrates the ad, framing the campaign as a battle to protect Americans’ individual liberties to the sound of Beyonce’s song Freedom.

The next highly anticipated development will be Harris’ choice of a vice-presidential candidate to counter Trump’s selection of US Senator JD Vance of Ohio.

The list of contenders amounts to a who’s-who of rising Democrats, including US Senator Mark Kelly of Arizona, governors Josh Shapiro of Pennsylvania, Roy Cooper of North Carolina and Andy Beshear of Kentucky, as well as Transportation Secretary Pete Buttigieg.

– with AAP