Former US President Donald Trump spoke on Saturday at a rally in Nevada, where he said he feels "sharper now than I did 20 years ago," as he reacted to recent attacks on his age and verbal gaffes by Republican presidential rival Nikki Haley, Reuters reported.

Trump also said presidential candidates should have to take a cognitive test, in an apparent response to a challenge from Haley, who has advocated the same policy, citing the age of Trump, 77, and President Joe Biden, 81.

Haley’s attacks on Trump follow recent verbal slip-ups by the former President. During a speech on January 19, he confused Haley with former House Speaker Nancy Pelosi. On occasion he has appeared to slur his words and he also suggested former President Barack Obama was still in office.

Trump won the New Hampshire primary this week, a week after winning the Iowa caucuses. While those wins have all but assured him his party's White House nomination, he is infuriated that Haley has refused to drop out.

After losing New Hampshire, Haley congratulated Trump on his victory but clarified she has no intention of stepping down.

“New Hampshire is first in the nation. It is not the last in the nation. This race is far from over. There are dozens of states left to go, and the next one is my sweet state of South Carolina,” she declared.

Senator Ted Cruz (R-TX), who has endorsed Trump, predicted on Wednesday that the GOP primary race will effectively “end” in South Carolina after Trump wins that primary, prompting Haley to drop out.

“My prediction right now is Nikki will drop out 18 hours after South Carolina. After that, you cannot go on,” he said.