Republican presidential candidate Donald Trump has claimed that his Democratic rival Kamala Harris has a "bigger cognitive problem" than President Joe Biden, who pulled out of the White House bid after a disastrous debate performance.

Fears over 81-year-old Biden's age and mental state grew after the June 27 debate between him and Trump.

Addressing an election rally in the battleground state of Georgia, Republican Party presidential candidate Trump said the world is "laughing" at 59-year-old Vice President Harris.

“You know what they're really laughing at? Kamala, because they can't believe that she's going to be president. They can't believe it," Trump, 78, said.

"You talk about cognitive problems? She's got bigger cognitive problems than he (Biden) has, in my opinion,” Trump alleged.

“This is how we're going to end the era of inflation and mayhem, misery. Under Kamala and Crooked Joe, we had such misery. Think about what we've been through with inflation, with the wars all over the world,” he said.

Talking about Hungary's Prime Minister Viktor Orban, with whom he shares close ties, Trump said, “...Orban, strong man, really a great leader. Didn't allow anybody into his country. They said, 'Why is the world blowing up? The Middle East is blowing up. Russia looks like it's going to end up in World War III. All of these problems are happening. They weren't here',” he said.

“He (Orban) said, it's one simple reason. Donald Trump isn't president. You make Donald Trump president of the United States. Nobody understood him. Everybody was afraid of him. They feared Donald Trump...China feared him. Russia feared him. They all feared him. We had no problems,” Trump said.

“Russia was never going into Ukraine, and they didn't. For four years, they didn't. When I left, they just went in," he said.

Trump alleged that if Harris got four more years, she would de-industrialise the US and destroy the country.

“We will become virtually a banana republic. We will be destroyed. Their plans are horrible," he said.

The presidential elections will take place in the US on November 5.

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