Former Trump administration officials said they repeatedly warned the president that claims that Ukraine was responsible for hacking the 2016 US election were groundless, The New York Times reported on Sunday.

The report backs up an interview with President Donald Trump's former homeland security adviser Tom Bossert, who told ABC that Trump's lawyer Rudy Giuliani and others persisted with the theory even after White House officials debunked it.

Democrats last week launched an impeachment inquiry after a whistleblower detailed concerns that Trump had sought to pressure Ukraine to investigate his political rival Joe Biden.

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Several former White House officials said they told President Donald Trump that a conspiracy theory that Ukraine was secretly responsible for hacking the 2016 US presidential election was false, The New York Times reported on Sunday.

According to the paper, their warnings had little effect on the president, who pursued the theory anyway.

The report backs up a claim by the former homeland security adviser Tom Bossert, who earlier Sunday attacked Trump's lawyer Rudy Giuliani for keeping the flawed theory alive and said he'd warned Trump that it was groundless.

"It's not only a conspiracy theory, it is completely debunked," Bossert told ABC'S George Stephanopoulos on "This Week."

The elaborate conspiracy theory, long circulated in right-wing media, alleges that it was really Ukraine that was behind interference in the 2016 election and that it attempted to pin the blame on Russia as part of a secret plot to help Democrats.

Other former presidential aides told The Times that they had struggled to convince Trump that Russia, not Ukraine, was behind the election interference.

The outlet said Trump "was more willing to listen to outside advisers" like Giuliani "than his own national security team."

One former aide told the publication that Giuliani would "feed Trump all kinds of garbage" that created "a real problem for all of us."

A former senior official told The Times that Bossert repeatedly told Trump that his pet theory — that a Democratic National Committee computer server was stashed in Ukraine to conceal evidence of its involvement in election hacking — was groundless.

In his ABC interview, Bossert blamed Giuliani for pushing the theory with the president.

"At this point, I am deeply frustrated with what he and the legal team is doing in repeating that debunked theory to the president," he said. "It sticks in his mind when he hears it over and over again. And for clarity here, George, let me just, again, repeat that it has no validity."

Rudy Giuliani and Trump. Mike Segar/Reuters

Trump is facing one of the gravest crises of his presidency after a whistleblower's complaint that the president attempted to pressure Ukraine to investigate his Democratic rival Joe Biden led House Democrats to launch an impeachment inquiry.

The whistleblower's complaint said Trump also requested help from his Ukrainian counterpart, President Volodymyr Zelensky, in investigating the conspiracy theory tying Ukraine to the election interference.

It also said Giuliani's associates met with Ukrainian officials about the conspiracy theory, claiming to be working to supply evidence for a Justice Department inquiry into the origins of the FBI's Russia investigation.

Giuliani appeared on ABC after Bossert and pushed back on the former official's claims, denying that he ever promoted the conspiracy theory.

"With all due respect to Tom Bossert, he doesn't know what he's talking about," Giuliani said.

The former New York mayor told Politico last week that he was tipped off by a private investigator late last year about Ukrainian interference in the 2016 election, leading him on a mission to investigate the claim.