Joe Biden has lashed out at US President Donald Trump, saying he could "beat him like a drum" and demanded a probe into allegations that the president tried to bully Ukraine into joining a "smear" aimed at him.
"Trump's doing this because he knows I'll beat him like a drum," Mr Biden told reporters on the campaign trail in Iowa on Saturday.
"He's using the abuse of power and every element of the presidency to try to do something to smear me."
The Democratic front-runner also wagged his finger at a reporter who asked whether he intervened with Ukraine on behalf of his son, Hunter, while serving as vice president.
Mr Biden lectured the reporter to focus on Mr Trump's reported abuse of power, which is the subject of a controversial whistleblower complaint that the White House is trying to squelch.
"You should be looking at Trump," Mr Biden scolded. "Everybody looked at this and everybody who's looked at it said there's nothing there. Ask the right question."
Shaking hands with supporters at the Iowa Steak Fry in Des Moines, Mr Biden boasted he would not hesitate to punch back in a general election dog fight with Trump.
"I know what I'm up against, a serial abuser. That's what this guy is. He abuses power any way he can," Mr Biden said. "This crosses the line."
The comments from Mr Biden came as Mr Trump continued to defend himself against claims that he has abused his power.
Published reports say Mr Trump repeatedly pressured Ukraine President Volodymyr Zelensky to open a probe into Biden and his son, even though they have already been cleared of any impropriety.
Although the substance of the whistleblower complaint has not been officially revealed, Mr Trump insisted that the 25 July call was a "perfectly fine and routine conversation".
"Nothing was said that was in any way wrong," Mr Trump said.
Mr Biden and other Democrats are demanding that the White House release a transcript of the call, in which The Wall Street Journal says Mr Trump asked Mr Zelensky eight times about the Bidens.
Joe Biden. Source: The New York Times
It would be a serious abuse of power if Mr Trump were found to have used his position to push a foreign leader to tarnish a political opponent.
Mr Trump put a hold on $US250 million ($A369 million) of security assistance to Ukraine around the same time, but reports say he did not specifically threaten to withhold the money in the call with Mr Zelensky.
The money has since been released.
Ukrainian officials appear to be caught in a tough spot by the controversy.
They need to stay in favour with the US to maintain aid that helps the country defend itself against Russian aggression. But they do now want to appear to be taking orders from Mr Trump.
Ukraine Foreign Minister Vadym Prystaiko told a local news outlet there was no "pressure" in the call and said Ukraine does not want a transcript made public.
"Leaders have the right to discuss any existing issues," he said. "This was a long and friendly conversation that touched on a lot of issues, sometimes requiring serious answers."
