Trump's own staff repeatedly warned him that his theory about Democrats and Ukraine had been debunked

BRENDAN SMIALOWSKI/AFP/Getty Images

US President Donald Trump makes a statement for the press before a meeting with Ukraine's President Petro Poroshenko at the Palace Hotel during the 72nd United Nations General Assembly September 21, 2017 in New York City.

• Former Trump administration officials told The New York Times that they warned the president repeatedly that claims that Ukraine was responsible for hacking the 2016 election were groundless.
• It appears to back up an interview from former Homeland security adviser, who told ABC News that 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 warned that Trump had sought to influence Ukraine to investigate his political rival Joe Biden.
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Several former White House officials told President Donald Trump that a conspiracy theory alleging that Ukraine was secretly responsible for hacking the 2016 presidential election was false, The New York Times reported.

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

Their testimony backs up a claim made former national security adviser Tom Bossert, who over the weekend attacked Trump's lawyer Rudy Giuliani for keeping the flawed theory alive and said he warned Trump that it was groundless.

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

He was talking about an elaborate conspiracy theory that has long circulated in right-wing media, alleging that it was really Ukraine that was behind interference in the 2016 presidential election.

According to the theory, Ukraine attempted to pin blame for the interference on Russia as part of a secret plot to help Democrats.

Other former presidential aides later, backing Bossert's claim, told The New York Times that they had struggled to persuade Trump that Russia, and not Ukraine, was behind election interference.

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

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

One former official told the Times that Bossert repeatedly told Trump that his pet theory - that a DNC computer server had been stashed in Ukraine to conceal evidence of its involvement in election hacking - was groundless.

Bossert had in his ABC interview singled out Trump's personal attorney, Rudy Giuliani, as being to blame for pushing the theory to the president.

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

Mike Segar/Reuters

Rudy Giuliani and Donald Trump

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

According to the whistleblower's complaint, Trump also requested that his Ukrainian counterpart, President Volodymyr Zelensky, help in attempts to tie Ukraine to 2016 election interference.

The whistleblower also alleges in the complaint that Giuliani also met Ukrainian officials about the Ukraine election interference conspiracy theory, claiming to be working to supply evidence for a Justice Department probe into the origins of the FBI's Russian interference investigation.

Giuliani appeared on ABC after Bossert, and pushed back against the former official's claims, denying ever promoting the conspiracy.

"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 had been tipped off by a private investigator at the beginning of the year about Ukrainian interference in the 2016 election, leading him on a mission to investigate the claim.