A former Homeland Security adviser in the Trump administration said Sunday that the unsubstantiated claim that Ukraine was responsible for the hack of the Democratic National Committee (DNC) in 2016 is a "conspiracy theory" with "no validity."

Tom Bossert, who served in the administration between 2017 and 2018, made the comments on ABC's "This Week" while condemning Trump's personal attorney 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 for "repeating the debunked theory" related to Ukraine and the DNC server to the president.

The comments from Bossert came in light of revelations that Trump addressed the hack of the DNC during a July 25 phone call with Ukrainian President Volodymyr Zelensky.

"It’s not only a conspiracy theory. It is completely debunked," Bossert said, adding that he communicated this to Trump during his time working in the administration.

"At this point, I am deeply frustrated with what [Giuliani] and the legal team are doing and repeating that debunked theory to the president," Bossert added. "It sticks in his mind when he hears it over and over again, and for clarity let me just repeat that it has no validity.

NEW: Former Homeland Security adviser Tom Bossert on impeachment inquiry against Pres. Trump: "It is a bad day and a bad week for the president and for this country if he is asking for political dirt on an opponent." https://t.co/tXuhTOmM7P pic.twitter.com/ab1HCFVjhD — This Week (@ThisWeekABC) September 29, 2019

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"The United States government reached its conclusion on attributing to Russia the DNC hack in 2016 before it even communicated it to the FBI, long before the FBI ever knocked on the door at the DNC," Bossert said.

Giuliani pushed back just moments later while appearing on ABC, saying that Bossert "doesn't know what he's talking about."

Trump is facing mounting scrutiny over revelations that he sought to enlist Ukraine's help investigating 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 and Biden's son. A whistleblower complaint released last week accuses Trump of repeatedly pressuring the leader of Ukraine to open an investigation into the Bidens over unsubstantiated allegations of corruption.

A White House memo of the leaders' July 25 phone call confirms Trump asked Zelensky to work with Giuliani and Attorney General William Barr Bill BarrFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Why a backdoor to encrypted data is detrimental to cybersecurity and data integrity FBI official who worked with Mueller raised doubts about Russia investigation MORE to investigate the Bidens.

It also shows that Trump called on Zelensky to look into matters related to CrowdStrike, a U.S.-based internet security company that initially examined the breach of the DNC's servers in 2016, after the Ukrainian leader asked about buying U.S. anti-tank missiles. CrowdStrike determined in 2016 that Russian agents broke into the DNC's network and stole emails that were later released by WikiLeaks.

The request was an apparent reference to a conspiracy theory that casts doubt on the assessment that Russians were responsible for hacking the DNC.

The latest revelations regarding Trump led 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.) to announce a formal impeachment inquiry of the president. Trump has responded by repeatedly railing against Democrats and the media.

He has also questioned the loyalty of the whistleblower, saying last week at a private event that the whistleblower and the White House officials who gave the figure information were "almost" spies, according to a recording obtained by the Los Angeles Times.