Joe Biden has called for an investigation into Donald Trump after a whistleblower reportedly warned officials that the US president had urged his Ukrainian counterpart to launch a probe into the former vice president’s son.

Speaking to reporters on Saturday about the unprecedented controversy, Mr Biden, one of the frontrunners for the 2020 Democratic presidential nomination, said: “Here’s what I know: I know Trump deserves to be investigated.”

Mr Trump “is violating every basic norm of a president”, he added. “You should be asking him the question, why is he on the phone with a foreign leader, trying to intimidate a foreign leader, if that’s what happened.”

Mr Biden went on to address all of the reporters standing before him in Des Moines, Iowa, where he was participating in the Polk County Steak Fry along with other leading 2020 Democratic candidates.

“You should be looking at Trump,” he said, pointing his fingers at several journalists. “Trump’s doing this because he knows I’ll beat him like a drum, and he’s using the abuse of power and every element of the presidency to try to do something to smear me.”

The comments arrived amid growing concern on Capitol Hill and across the country over Mr Trump’s reported phone call with Ukrainian president Volodymyr Zelensky in July, in which he is alleged to have repeatedly demanded an investigation into Hunter Biden, who held lucrative contracts in Ukraine while his father was vice president.

Mr Trump has claimed the younger Mr Biden’s business dealings were propped up by his father’s position. He reportedly asked the Ukrainian president to launch an investigation “about eight times”, the Wall Street Journal reported.

“Everybody’s looked at this and said there’s nothing there,” Mr Biden said on Saturday, adding: “Ask the right questions.”

A previous statement from Mr Biden’s campaign was far more reserved than the former vice president appeared on Saturday.

If the reports were true, Mr Biden said, “then there is truly no bottom to President Trump’s willingness to abuse his power and abase our country”.

“This behaviour is particularly abhorrent because it exploits the foreign policy of our country and undermines our national security for political purposes,” he said. “It means that he used the power and resources of the United States to pressure a sovereign nation – a partner that is still under direct assault from Russia – pushing Ukraine to subvert the rule of law in the express hope of extracting a political favour.”

He added: “Such clear-cut corruption damages and diminishes our institutions of government by making them tools of a personal political vendetta.”

Mr Biden and others have also called on the president to release a full transcript of the phone call in question.

Mr Trump defended himself against accusations of calling on a foreign government to interfere in the upcoming election, attacking Mr Biden in a series of angry morning tweets over the weekend and again turning the focus onto his potential 2020 opponent.

“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, while at the same time trying to protect Sleepy Joe Biden,” he tweeted, adding: “Will fail again!”