Former President Donald Trump accused both Biden, the Democrat he will likely face in November's general election, and Nikki Haley, Trump's last remaining rival for the Republican presidential nomination, of lacking the mental capacity to be president.

Haley - like Trump, campaigning in South Carolina where the two will meet in a primary election on Feb. 24 - went after both men, calling Trump mentally deficient and saying Biden is too old to be president.

Meanwhile, the Biden White House, responding to the report on Thursday from a Department of Justice special counsel that said Biden had a poor memory, continued its full-scale attack on Trump's age and mental acuity after Trump recently mixed up names and made other verbal gaffes.

"Every single time Donald Trump opens his mouth, he's confused, deranged, lying, or worse," T.J. Ducklo, a Biden spokesman, said in a statement released by Biden's reelection campaign.

The issue of mental competency has become a major topic in this year's presidential campaign. Biden, 81, and Trump, 77, are the two oldest men respectively to have been elected president. In recent days, Biden has mixed up the names of some world leaders.

The issue is a vexing one for Biden's reelection campaign. In a Reuters/Ipsos poll conducted in September, 77% of respondents said they agreed with a statement that Biden was too old to work in government, while 56% said the same of Trump.

Haley, 52, has called for mental competency tests for presidential candidates over 75 years old.

The issue was thrust front and center again after Special Counsel Robert Hur, a former U.S. attorney in Maryland during Trump's administration, said in his report that he chose not to bring criminal charges against Biden following a 15-month investigation into his handling of classified documents because the president cooperated.

Hur said the Democratic incumbent would be difficult to convict and described him as a "well-meaning, elderly man with a poor memory" who was not able to recall to investigators when his son, Beau Biden, died.

Biden angrily denied Hur's allegations about his memory, saying in a White House appearance on Thursday night, "my memory's fine."

Trump, at a rally in Conway, South Carolina, said Hur's report showed Biden "is not fit to serve as our commander in chief."

Trump - who faces four state and federal criminal trials, including one for mishandling classified documents - is close to clinching the Republican nomination, and the prospect of a likely general election rematch with Biden in November.

Haley, who has no clear path to the nomination after Trump's consecutive wins in Iowa, New Hampshire and Nevada, is refusing to quit the race, making a potential last stand in her home state of South Carolina, where she trails badly in opinion polls behind Trump.

Trump on Saturday called his former U.N. ambassador "birdbrain" and "brain-dead," suggesting she did not have the mental capacity to enter the White House.

Haley, beginning a two-week bus tour of South Carolina, called Biden "diminished." She also cited a recent Trump speech where he confused her with former Democratic House of Representatives Speaker Nancy Pelosi.

"It's bigger than just Joe Biden. Whether it's Donald Trump getting me confused with Nancy Pelosi ... it's time for a new generational leader," Haley told reporters.

(Reporting by Tim Reid in Washington; Additional reporting by Jason Lange; editing by Jonathan Oatis)

By Tim Reid