Donald Trump said on Tuesday that he will not become a dictator if he becomes U.S. president again except "on day one", after warnings from Democrats and some Republicans that America was in danger of becoming an autocracy if he wins the 2024 election.

Republican presidential candidate Trump had to be asked twice during a televised town hall event in Iowa to deny that he would abuse power to seek revenge on political opponents if re-elected to the White House.

"No. No. Other than day one," Trump said when asked to deny he would become a "dictator" if he wins the November election.

Trump said on "day one" he referred to, he would use his presidential powers to close the southern border with Mexico and expand oil drilling.

Trump, seeking a second White House term in a likely election re-match with Democratic President Joe Biden, has frequently promised "retribution" on political opponents if he gains power again.

Targets include Biden, prosecutors who have charged him with dozens of crimes, the Department of Justice, and the federal bureaucracy, he said in campaign speeches and TV appearances this year.

Trump, the frontrunner for the Republican presidential nomination, was appearing at a Fox News event before a friendly audience in Davenport, Iowa, the state where the party's nominating contest kicks off on Jan. 15.

As soon as the event finished, Biden's campaign manager, Julie Chavez Rodriguez, said in a statement: "Donald Trump has been telling us exactly what he will do if he's re-elected and tonight he said he will be a dictator on day one. Americans should believe him."

Trump was U.S. president between 2017 and 2021 and has refused to concede that he lost to Biden in the 2020 election.

Since then Trump has spread false claims that the 2020 election was stolen from him, a conspiracy that fueled the deadly insurrection by Trump supporters at the U.S. Capitol on Jan. 6, 2021. Trump's election lies also form a cornerstone of his current White House campaign.

Trump's rivals for the nomination, including Florida Governor Ron DeSantis and former U.S. Ambassador to the U.N. Nikki Haley, will appear at a televised debate on Wednesday at the University of Alabama at 7 p.m. CST (0100 GMT).

Trump will skip the event, as he has done for the three previous Republican debates.

Biden has repeatedly warned that Trump is a threat to democracy and that a second Trump term could usher in an unprecedented and dangerous age of American autocracy.

Former U.S. Representative Liz Cheney, a Republican who is an outspoken critic of Trump and who co-chaired the congressional probe of the attack on the Capitol, said in media interviews to promote a memoir this week that a Trump dictatorship is a "very real threat" if he wins re-election.