Donald Trump said on Wednesday that his recent mixing up of names was intentional.

During a speech last month, Trump confused Nikki Haley, his last remaining rival for the Republican presidential nomination, with former Democratic House Speaker Nancy Pelosi. He also suggested former Democratic President Barack Obama was still in office.

"When I purposely interposed names, they said I didn't know Pelosi from Nikki," Trump said at a rally in South Carolina on Wednesday. He said calling Obama the current president was sarcasm, not a gaffe.

"I'm a great speaker," Trump said.

The issue of age and mental competence have been thrust to the forefront of the 2024 election campaign after a report last week suggested US President Joe Biden, 81, was suffering from memory lapses.

Meanwhile, Biden allies have pointed to verbal flubs by former President Trump, including the Haley and Pelosi mix up and the Obama remark.

Trump, the Republican frontrunner for his party's presidential nomination, is eying a general election rematch with Biden in November.

Haley, 52, has called for mental competency tests for presidential candidates over 75 years old, including Biden and Trump, the two oldest men elected to the U.S. presidency.

Some 78 per cent of respondents in a Reuters/Ipsos poll published Tuesday - including 71% of Democrats - think Biden is too old to work in government. Trump suffers less from voter skepticism over his age; 53 per cent of respondents consider him to be too old for government work.

Biden recently mixed up the names of world leaders, including France's former president Francois Mitterrand with its current president, Emmanuel Macron.

Special Counsel Robert Hur, a Republican former U.S. attorney in Maryland during Trump's administration, said in his report on Biden's handling of classified documents last week that Biden was a "well-meaning, elderly man with a poor memory" who was not able to recall to investigators when his son, Beau Biden, died.

Biden responded angrily to the report, saying his memory is "fine".

During Wednesday's rally, Trump also insisted that he "loves" the U.S. military. Haley has attacked him in recent days for past comments Trump has made disparaging veterans, including one last week when he mocked her husband, who is deployed oversees with the Army National Guard. Trump is close to clinching the Republican nomination after wins in Iowa, New Hampshire and Nevada.

Haley, who has no clear path to the nomination, is refusing to quit the race, making a potential last stand in her home state of South Carolina in its Feb. 24 primary. She trails badly in opinion polls behind Trump there.