Joe Biden accused Donald Trump of abusing his power as the row over the former vice-president’s son’s business dealings in Ukraine escalated over the weekend.
Mr Biden sought to turn the tables on the US president and prevent the controversy over his son, Hunter, derailing his bid for the nomination.
His intervention came as the latest opinion poll in Iowa showed that he had fallen behind Massachusetts senator Elizabeth Warren in the race to be the party’s presidential candidate next year.
Even the slightest hint of impropriety could prove hugely damaging to Mr Biden, whose often halting performances on the campaign trail have seen his status as the Democratic front-runner eroded in recent weeks.
“The real story involves Hunter Biden going around the world and collecting large payments from foreign governments and foreign oligarchs.” Peter Schweizer Laura Ingraham Hunter made a fortune in Ukraine and in China. He knew nothing about Energy, or anything else.— Donald J. Trump (@realDonaldTrump) September 22, 2019
The controversy, which has intensified the long-running feud between Mr Trump and the former vice-president,centres on Hunter Biden’s involvement with Burisma, Ukraine’s natural gas producer, which was being investigated by Ukrainian prosecutor general Viktor Shokin.
Mr Trump and his supporters allege that as vice-president, Mr Biden put pressure on Ukraine to sack Mr Shokin to stymie the investigation, an accusation which was angrily denied.
Mr Shokin’s successor has since said there was no evidence of wrongdoing by Hunter Biden.
Speaking in Iowa, Mr Biden told reporters: “I have never spoken to my son about his overseas business dealings.”
According to the Wall Street Journal, Mr Trump did so “about eight times”.
It is alleged the US president used a delayed package of military aid to put pressure on Mr Zelensky in what has been seen as an attempt to persuade a foreign government to effectively intervene in the 2020 US presidential election.
This alarmed an intelligence service whistleblower, who raised the issue.
However, the White House has resisted attempts by the Democrat-controlled House Intelligence Committee to see details of the whistleblower's complaint.
“Here’s what I know. Trump should be investigated,” Mr Biden said in Iowa.
“He’s doing this because he knows I’ll beat him like a drum. And he’s using an abuse of power and every element of the presidency to try to smear me ... Ask the right questions.
“This appears to be an overwhelming abuse of power. We have never seen anything like this from any president.”
Mr Trump remained defiant, insisting his conversation with Mr Zelensky was “perfectly fine and routine.”
He wrote on Twitter: “Now that the Democrats and the Fake News Media have gone ‘bust’ on every other of their Witch Hunt schemes, they are trying to start one just as ridiculous as the others, call it the Ukraine Witch Hunt.”
The US president continued to defend the call speaking to reporters on Sunday before leaving Washington for events in Texas and Ohio.
“The conversation I had was largely congratulatory, was largely corruption, all of the corruption taking place,” he said.
"It was largely the fact that we don’t want our people, like Vice President Biden and his son, creating to the corruption already in the Ukraine.”
If the President asked or pressured Ukraine’s president to investigate his political rival, either directly or through his personal attorney, it would be troubling in the extreme. Critical for the facts to come out.— Mitt Romney (@MittRomney) September 22, 2019
However, one senior Republican, Utah senator and former GOP presidential candidate, Mitt Romney, appeared to break ranks with Mr Trump.
Mr Romney wrote on Twitter that it would be “troubling in the extreme” if Mr Trump had pressured the Ukrainian president to investigate his political rival.