Republican presidential candidate Nikki Haley has questioned whether Donald Trump is mentally capable of serving a second term after he repeatedly seemed to confuse her with former US House speaker Nancy Pelosi.

As she campaigned in Keene, New Hampshire, on Saturday, Ms Haley referenced Mr Trump’s speech the night before, in which he mistakenly asserted she was in charge of Capitol security on January 6 2021, when a mob of the former president’s supporters stormed the building seeking to stop the certification of his loss to Democrat Joe Biden.

Mr Trump first said Ms Haley turned down security offered by his administration on January 6 and then again mentioned her, adding, “They destroyed all of the information, all of the evidence, everything, deleted and destroyed all of it.”

The former president, 77, has accused Mrs Pelosi of turning down security he says his administration offered, but a special House committee investigating the attack found no evidence to support that claim.

Ms Haley said: “They’re saying he got confused, that he was talking about something else, he’s talking about Nancy Pelosi. He mentioned me multiple times in that scenario.

“The concern I have is — I’m not saying anything derogatory — but when you’re dealing with the pressures of the presidency, we can’t have someone else that we question whether they’re mentally fit to do this,” Ms Haley said.

Ms Haley’s campaign manager Betsy Ankney said Mr Trump “made a pretty apparent gaffe” but the former president’s campaign senior adviser told reporters: “It’s Nikki and Nancy. What’s the difference?”

At his rally in Manchester on Saturday, Mr Trump said he took a cognitive test and “aced it”.

“I’ll let you know when I go bad. I really think I’ll be able to tell you,” he said. “I feel my mind is stronger now than it was 25 years ago. Is that possible?”

Since entering the Republican race nearly a year ago, Ms Haley, 52, has advocated for “mental competency tests” for older politicians, a swipe at the ages of both Mr Trump and President Biden.