Tom Bossert, a former homeland security adviser to President Donald Trump , said a conspiracy theory pushed by Rudy Giuliani, the presidents personal attorney, that Ukraine was behind the 2016 cyberattack against the Democratic National Committee is completely false.

Bossert, who joined ABC News as an on-air analyst after resigning from the administration in April 2018, said Sunday that hes deeply frustrated that Giuliani continues to peddle the unsubstantiated theory.

Its not only a conspiracy theory, it is completely debunked, Bossert told ABCs George Stephanopoulos during Sundays episode of This Week.

I am deeply frustrated with what [Giuliani] is doing in repeating that debunked theory to the president, he continued.

The conspiracy theory was drawn into the national spotlight last week after the White House released a summary of Trumps July phone call with Ukrainian President Volodymyr Zelensky.

During the call, which would become the focus of a U.S. intelligence officials subsequent whistleblower complaint, Trump allegedly pressured Zelensky to investigate former Vice President Joe Biden as well as Crowdstrike, the cybersecurity company that investigated the DNC hack.

Giuliani has claimed that Crowdstrike falsely accused the Russians of carrying out the cyberattack and that Ukraine was actually behind it. Theres no evidence to support these allegations. Multiple U.S. intelligence agencies have determined Russia was responsible for the hack.

It has no validity, Bossert said of the Ukraine-DNC conspiracy theory. The United States government reached its conclusion on attributing to Russia the DNC hack in 2016 before it even communicated it to the FBI .

Bossert said hes deeply disturbed by Trumps call with Zelensky, which Democrats say amounts to a U.S. president seeking foreign assistance in his bid for reelection.

It is a bad day and a bad week for this president and this country if he is asking for political dirt on an opponent, Bossert told ABC. He added, however, that hes not totally convinced Trump delayed military aid to Ukraine for the purpose of persuading Ukraine to meet his political demands.

Stephanopoulos asked Bossert whether its appropriate for Ukraine to investigate a debunked conspiracy theory and to investigate a political rival of a foreign leader.

The DNC server and that conspiracy theory has got to go, Bossert said. They have to stop with that. It cannot continue to be repeated in our discourse.

He warned Trump against continuing to focus on past grievances from Robert Muellers Russia investigation.

If he continues to focus on that white whale, its going to bring him down, Bossert said. Enough.