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.
In a Fox News event, Mr Trump did not rule out abusing power if he returns to the White House. The Republican also said he thought Mr Biden was too old for such a political rematch.
Neither candidate is yet the official nominee of their respective party.
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.
Current opinion polls indicate a hypothetically tight race between the two as the 2024 White House campaign heats up.
Speaking to reporters off camera at a campaign fundraising event near Boston, Mr Biden said: “If Trump wasn’t running, I’m not sure I’d be running. But we cannot let him win.”
The president’s remarks were regarded as the first time he has pinned his aspirations for a second term squarely on opposing Mr Trump’s comeback bid.
Later on Tuesday, Mr Trump appeared at a televised Fox News town hall in Davenport, Iowa.
The host, Sean Hannity, asked: “You are promising America tonight, you would never abuse power as retribution against anybody?”
“Except for day one,” Mr Trump replied. “I want to close the border and I want to drill, drill, drill.”
Earlier in the interview, Mr Hannity asked the former president if he “in any way” had “any plans whatsoever, if re-elected president, to abuse power, to break the law to use the government to go after people”.
“You mean like they’re using right now?” Trump replied.
