At the center of the complaint, according to media reports, is Trump's phone call in July with Ukrainian President Volodymyr Zelensky, in which Trump reportedly pressed his Ukrainian counterpart to seek information about business dealings between Biden's son, Hunter Biden, and a Ukrainian gas company. The information could potentially be damaging to Biden senior's White House bid.
"You should be asking him (Trump) the question: why is he on the phone with a foreign leader, trying to intimidate a foreign leader?" Biden was quoted by Politico as saying Saturday in Des Moines, Iowa.
"This appears to be an overwhelming abuse of power. To get on the phone with a foreign leader who is looking for help from the United States and ask about me and imply things ... this is outrageous. You have never seen anything like this from any president," said the former vice president.
Asked by reporters to comment on the whistleblower complaint, which was filed by an unidentified intelligence official in mid-August, Trump said Friday in the Oval Office that the revelation is "partisan" and "just another political hack job."
Claiming that his phone call with Zelensky was "a beautiful conversation," the president reiterated the need to probe his potential competitor in the 2020 election.
"Somebody ought to look into Joe Biden's statement, because it was disgraceful, where he talked about billions of dollars he's not giving to a certain country, unless a certain prosecutor is taken off the case," he said.
According to an exclusive report by the Wall Street Journal on Friday, Biden, during his trips to Ukraine as vice president, had asked the Ukrainian government to oust former prosecutor general, Viktor Shokin, who had investigated Burisma Group, a private Ukrainian gas company of which Biden junior was a board member during his father's vice presidency.
The Journal said Trump's attorney Rudolph Giuliani suggested that the former vice president's motivation was to protect his son, although Shokin had already completed his investigation of Burisma Group before he left office.
Biden on Saturday resolutely denied having spoken with his son about the latter's overseas business dealings. "You should be looking at Trump," Biden said, according to Politico. "Everybody looked at this and everybody who's looked at it said there's nothing there. Ask the right question."
On the election, Biden said: "I know what I'm up against, a serial abuser. That's what this guy is. He abuses power any way he can."
Trump slammed the Democrats and the media on Saturday, tweeting that they started what he termed "the Ukraine Witch Hunt" and tried to "protect Sleepy Joe Biden."
Biden, for his part, said in Des Moines that Trump has been "using every element of his presidency to try and smear me."