US President Joe Biden has said part of his motivation to run for a second term next year is to prevent the return of Donald Trump.
"We cannot let him win," Mr Biden told reporters.
At a Fox News event, Mr Trump said that he thought Mr Biden was too old for such a political rematch.
Mr Trump is the runaway front-runner in the Republican race even as he faces four criminal cases. Mr Biden, as the incumbent president, is the presumptive Democratic nominee.
But neither candidate is yet the official nominee of their respective party.
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