Former Vice President 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 said on Wednesday there was "zero, zero, zero evidence" behind 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 allegations regarding the 2020 candidate's behavior with Ukraine.

Trump is claiming Biden used his influence as vice president to push Ukraine to fire a senior prosecutor who at one point had investigated an energy company where Biden's son, Hunter, had served as a board member.

"There is zero, zero, zero, zero evidence of any assertion being made," Biden said at a 2020 gun safety forum in Las Vegas.

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"Look, the issue is this president of the United States engaged in something apparently that is close to, well, engaged in activity which at minimum gives a lot of running room for the Russians and Ukraine, and I think we should just focus on he's the issue," he continued. "Nobody has ever asserted that I did anything wrong except he and what’s that fella’s name? Rudy ... Giuliani. That’s it."

Trump has stepped up his attacks on Biden over the past week, releasing an ad on Friday that zeroes in on the allegations.

Biden has said the Obama administration advocated for the prosecutor's removal because it believed he was not doing enough to combat corruption in Ukraine.

Trump and Republicans are on the defensive after Speaker Nancy Pelosi Nancy PelosiDemocratic senator to party: 'A little message discipline wouldn't kill us' Overnight Health Care: New wave of COVID-19 cases builds in US | Florida to lift all coronavirus restrictions on restaurants, bars | Trump stirs questions with 0 drug coupon plan Overnight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds MORE (D-Calif.) announced last week that the House would launch a formal impeachment inquiry amid allegations from a whistleblower that Trump sought to persuade Ukraine to investigate Biden.