Democratic presidential candidate Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE dismissed President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE’s call for an investigation into the former vice president's ties to Ukraine amid conservatives’ accusations of misconduct.

“Not one single, credible outlet has given any credibility to his assertion. Not one single one,” Biden told reporters Friday. “So I have no comment except the president should start to be president.”

Joe Biden reacts to reports that President Trump repeatedly pressured Ukraine's President to investigate Biden's son: "Not one single credible outlet has given any credibility to (Trump's) assertion. ... The President should start to be President" https://t.co/Vyrfcgkqv1 pic.twitter.com/Qy7gb5mOZT — CNN Politics (@CNNPolitics) September 20, 2019

The comment comes after reports emerged Friday that Trump pressured Ukrainian President Volodymyr Zelensky during a phone call in July to work with Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE, his personal attorney, to investigate Biden’s son, Hunter.

Trump did not deny the reports on Friday, telling reporters, “it doesn’t matter what I discuss.”

ADVERTISEMENT

“But I will say this: Somebody ought to look into Joe Biden's statement, because it was disgraceful,” he added.

Trump and other Republicans have sought to cast Biden as corrupt over his efforts as vice president in 2016 to push Ukraine to dismiss a top prosecutor.

The prosecutor was investigating a natural gas company owned by a Ukrainian oligarch, and Hunter Biden sat on the company's board.

No evidence has emerged to suggest Biden acted with his son’s interests in mind, but Trump’s allies, including Giuliani, have said the former vice president should still be investigated.

The prosecutor eventually resigned amid corruption allegations.

Giuliani at first denied and then acknowledged that he had asked Ukraine to look into allegations against Joe Biden while appearing for an interview on CNN Thursday night.

Trump weighed in on the topic Friday amid mounting scrutiny over a whistleblower complaint from someone in the intelligence community that reportedly details a troubling "promise" he made to a foreign leader.

The New York Times reported Friday evening that Trump's desire for an investigation into Biden was part of the whistleblower complaint, while the Times and Washington Post have said the whistleblower’s claims specifically centered around Ukraine.

Joseph Maguire, the acting intelligence chief, is scheduled to testify publicly before Congress next week over the complaint. Trump is also slated to meet with Zelensky on the sidelines of the United Nations General Assembly next week, a senior administration official told reporters earlier Friday.