WASHINGTON — President Trump was repeatedly warned by his own staff that the Ukraine conspiracy theory that he and his lawyer were pursuing was “completely debunked” long before the president pressed Ukraine this summer to investigate his Democratic rivals, a former top adviser said on Sunday.
Thomas P. Bossert, who served as Mr. Trump’s first homeland security adviser, said he told the president there was no basis to the theory that Ukraine, not Russia, intervened in the 2016 election and did so on behalf of the Democrats. Speaking out for the first time, Mr. Bossert said he was “deeply disturbed” that Mr. Trump nonetheless tried to get Ukraine’s president to produce damaging information about Democrats.
Mr. Bossert’s comments, on the ABC program “This Week” and in a subsequent telephone interview, underscored the danger to the president as the House moves ahead with an inquiry into whether he abused his power for political gain. Other former aides to Mr. Trump said on Sunday that he refused to accept reassurances about Ukraine no matter how many times it was explained to him, instead subscribing to an unsubstantiated narrative that has now brought him to the brink of impeachment.
The latest revelations came as the impeachment inquiry rushed ahead at a brisk pace. The House chairman taking the lead said that the whistle-blower who brought the matter to light would testify soon and that a subpoena for documents would be issued early this week to Rudolph W. Giuliani, the president’s personal lawyer who spearheaded the effort to find dirt on Democrats in Ukraine. In a letter to the acting director of national intelligence, lawyers for the whistle-blower requested stepped-up efforts to ensure his safety, citing “serious concerns we have regarding our client’s personal safety.”
As Democrats pressed forward, a new poll showed that a majority of Americans supported an impeachment inquiry for the first time, a worrying development for a White House that until now has been able to make the argument that the public opposed impeaching Mr. Trump. A senior White House aide tried to turn the tables by arguing that Mr. Trump was the real whistle-blower because he was uncovering Democratic corruption.
As Republicans struggled to defend the president on Sunday, Mr. Bossert’s remarks offered a hint of cracks in the Republicans’ armor. While Mr. Bossert was forced out in 2018 when John R. Bolton became national security adviser, he has remained publicly loyal until now to a president who prizes fealty above all else.
“It is completely debunked,” Mr. Bossert said of the Ukraine theory on ABC. Speaking with George Stephanopoulos, Mr. Bossert blamed Mr. Giuliani for filling the president’s head with misinformation. “I am deeply frustrated with what he and the legal team is doing and 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 again repeat that it has no validity.”
He added that pressing Ukraine’s president was disturbing, but noted that it remained unproven whether Mr. Trump’s decision to withhold aid to Ukraine was tied to the demand for investigations into former Vice President Joseph R. Biden Jr. and other Democrats.
“It is a bad day and a bad week for this president and for this country if he is asking for political dirt on an opponent,” Mr. Bossert said. “But it looks to me like the other matter that’s far from proven is whether he was doing anything to abuse his power and withhold aid in order to solicit such a thing.” On Twitter on Sunday evening, he added that he did “not see evidence of an impeachable offense.”
Other former aides said separately on Sunday that the president had a particular weakness for conspiracy theories involving Ukraine, which in the past three years has become the focus of far-right media outlets and political figures. Mr. Trump was more willing to listen to outside advisers like Mr. Giuliani than his own national security team.
Mr. Trump has known Mr. Giuliani, the former New York mayor, for years and likes his pugnacious approach and the fact that he never pushes back, said one former aide, who like others asked not to be identified discussing internal matters. Mr. Giuliani would “feed Trump all kinds of garbage” that created “a real problem for all of us,” said the former aide.
House Democrats may try to explore that as they move expeditiously in their inquiry. Representative Adam B. Schiff, the chairman of the House Intelligence Committee, said on Sunday that the whistle-blower whose complaint rocked Washington last week would testify “very soon” and that Mr. Giuliani would be ordered to turn over documents.
Mr. Schiff, a former prosecutor who is the de facto chief of the inquiry, also issued a pointed warning to Mr. Trump and the White House, who have a history of blocking congressional requests for witnesses and records. “If they’re going to obstruct, then they are going to increase the likelihood that Congress may feel it necessary to move forward with an article of obstruction,” he said on “This Week.”
Mr. Trump continued his bellicose attacks on his accusers. “I want Schiff questioned at the highest level for Fraud & Treason,” he wrote on Twitter. And he threatened the whistle-blower, who is protected by law from retribution. “Was this person SPYING on the U.S. President? Big Consequences!”
Republicans have had a tough time defending Mr. Trump and have mostly tried to redirect the conversation to suggest that Mr. Biden engaged in wrongdoing. Representative Steve Scalise of Louisiana, the No. 2 Republican in the House, repeatedly changed the subject on Sunday when Chuck Todd, the moderator of NBC’s “Meet the Press,” pressed him on whether he believed a summary transcript of the Ukraine call merited further investigation.
“Well, they’ve been investigating President Trump for two years, making way for baseless allegations,” Mr. Scalise finally said. “They’re investigating everything.”
Senator Lindsey Graham, Republican of South Carolina, suggested that Mr. Trump appoint a special prosecutor to look into Mr. Biden’s role in the firing of a former prosecutor in Ukraine, and said he had no problem with the president’s phone call.
“I’m openly telling everybody in the country I have the president’s back because I think this is a setup,” he said on CBS’s “Face the Nation.”
One of the few Republicans to express concern over the allegations was Representative Will Hurd, Republican of Texas and a former C.I.A. officer who is not seeking re-election. “There are troubling issues within the whistle-blower’s report,” he said on “Face the Nation” on CBS. “But they are allegations. And I think that’s why we should explore these allegations through hearings.”
The White House put out Stephen Miller, the president’s senior adviser, to offer his defense on the Sunday talk show circuit. Appearing on “Fox News Sunday,” Mr. Miller denounced the whistle-blower as a “deep-state operative” who is part of a cabal of “unelected bureaucrats who think they need to take down this president.”
Mr. Trump, he added, was the one searching for wrongdoing by pursuing corruption allegations against Mr. Biden and Democrats. “The president is the whistle-blower here,” Mr. Miller said. “The president of the United States is the whistle-blower. And this individual is a saboteur trying to undermine a democratically elected government.”
Central to the complaint by the whistle-blower was a July 25 telephone call in which Mr. Trump pressed President Volodymyr Zelensky of Ukraine to “do us a favor” and investigate Democrats at a time when the president had just ordered $391 million in aid to Ukraine frozen.
While his focus on Mr. Biden has drawn the most attention, Mr. Trump also urged Mr. Zelensky to look into a theory about the 2016 election that holds that Ukraine hacked the Democratic National Committee and then framed Moscow, possibly at the behest of Democratic operatives.
He specifically cited an American cybersecurity firm, CrowdStrike, which he seemed to believe was a Ukrainian company, and brought up a Democratic National Committee computer server, which he suggested might be in Ukraine.
While serving Mr. Trump, Mr. Bossert repeatedly told him that his questions about the server were without merit, according to a former senior administration official. In fact, the main server for the committee was in the party’s headquarters in Washington, and was later displayed there, next to a file cabinet that was broken into by the Watergate burglars nearly a half-century ago.
The first time Mr. Bossert and other aides refuted the server theory came before the inauguration when intelligence agency directors briefed him on Russia’s election interference operation. Mr. Trump may not have absorbed it because he was thrown off guard when told about a Democratic-financed dossier that included unproven allegations about his ties to Russia.
Shortly before Valentine’s Day in 2017, Mr. Bossert brought in Mike Rogers, the director of the National Security Agency, to brief Mr. Trump not only on the summary about the conclusion that it was Russia, but with the technical mechanics that led to the conclusion. At that point, Trump appeared to register that it was Russia. But periodically after that, he would say at rallies that he wondered about the server. Mr. Bossert would not re-educate him each time.
Another former senior official said it was a constant struggle to convince Mr. Trump that Russia, not Ukraine, had interfered in the election. The president would accept it after speaking with his more grounded aides, this official said, but then revert to believing it was a plot by Democrats or Ukrainians or others after speaking with associates outside the administration like Mr. Giuliani.
But even as his role in the controversy was debated over the weekend, Mr. Giuliani had the endorsement of Mr. Trump to continue appearing on television on Sunday defending himself and the president, according to two Trump advisers.
“I am defending my client the best way I know how,” Mr. Giuliani said on “This Week,” appearing shortly after Mr. Bossert did.
In a brief telephone interview after his ABC appearance, Mr. Bossert allowed for the possibility that it was someone other than Mr. Giuliani who had gotten in Mr. Trump’s head.
“In fairness, I don’t know that it was Rudy Giuliani that put that conspiracy theory into the president’s head,” he said. “I know somebody did and I was under the impression it was Mayor Giuliani. If Mayor Giuliani wasn’t promoting the D.N.C. server conspiracy theory, then I apologize.”
But in his television interview, Mr. Bossert made clear how serious the issue was, suggesting it could end Mr. Trump’s presidency. “The D.N.C. server and that conspiracy theory has got to go; they have to stop with that,” he said. He noted that the president “has not gotten his pound of flesh yet” from the investigation into his own ties to Russia. “But George, if he continues to focus on that white whale, it’s going to bring him down.”
Reporting was contributed by Chris Cameron, Matthew Rosenberg, David E. Sanger, Julian E. Barnes and Michael S. Schmidt.