“The DNC server and that conspiracy theory has got to go,” he said, referring to unfounded claims that Ukraine stole Democratic National Committee emails in 2016 and then somehow framed Russia.

“It’s not only a conspiracy theory. It is completely debunked,” said Bossert, who served as homeland security adviser from January 2017 to April 2018. He added that if Trump continues with that focus, “it’s going to bring him down.”

Bossert’s comments stood in stark contrast to attempts by Trump’s allies to downplay a scandal that has sparked a formal impeachment inquiry, with the president’s personal attorney Rudy Giuliani and his senior adviser Stephen Miller both fiercely defending Trump on the Sunday news shows.

INTERACTIVE: See where House lawmakers stand on impeachment

Bossert singled out Giuliani for pushing the Ukraine conspiracy theory and disparaged the former New York City mayor for repeating it to the president.

Giuliani, who appeared on ABC after Bossert, denied ever supporting the theory, but he doubled down on a connection between Ukraine and Democrats, among other things.

“With all due respect to Tom Bossert, he doesn’t know what he’s talking about,” Giuliani said.

The competing narratives are playing out as information has leaked about Trump’s alleged attempts to undercut political rivals through his State Department, and his reported comments to Russian officials in 2017 that he wasn’t concerned about interference in the previous year’s election.

Three House committee chairmen on Friday subpoenaed Secretary of State Mike Pompeo for documents related to interactions between Trump, Giuliani and Ukrainian officials. They also issued a schedule of depositions for State Department officials named as playing a role in the Ukraine affair, which came to light because of a complaint from an anonymous whistleblower in the intelligence community.

Rep. Hakeem Jeffries of New York, the chairman of the House Democratic Caucus, said on Sunday that the impeachment inquiry would focus on “abuse of power,” adding that it could include possible White House failures to comply with congressional subpoenas.

With the House Intelligence, Foreign Affairs and Oversight committees moving ahead with their inquiry, Democrats have discussed the possibility of pursuing “obstruction of Congress” as part of potential articles of impeachment against Trump.

The Intelligence chairman, Rep. Adam Schiff (D-Calif.), has confirmed that the intelligence community inspector general, Michael Atkinson, will testify in a closed session before his panel at the end of this week. And on Sunday, Schiff said on ABC that he had reached a tentative agreement with the whistleblower and his attorneys to appear before the committee “very soon.”

“It is illegal, improper, a violation of oath, a violation of his duty to defend our elections and our Constitution for the president to merely ask for foreign interference in our election,” Schiff told Stephanopoulos.

Speaker Nancy Pelosi and her top deputies, during a private conference call on Sunday afternoon, laid out a strategy for their offensive against Trump, including plans to streamline their messaging operation and help vulnerable lawmakers who face potential repercussions in their districts.

Bossert said he was cautious about the impeachment proceedings after seeing “a lot of rush to judgment.” It’s far from proven, he said on Sunday, that Trump did anything to abuse his power and withhold aid to Ukraine in order to solicit its cooperation in an investigation of Biden, the former vice president and a leading candidate for the Democratic presidential nomination in 2020.

The president issued several tweets and retweets over the weekend attacking Democrats and the impeachment process, and also going after the whistleblower’s credibility.

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“The Whistleblower’s complaint is completely different and at odds from my actual conversation with the new President of Ukraine,” Trump wrote on Saturday. “The so-called ‘Whistleblower’ knew practically NOTHING in that those ridiculous charges were far more dramatic & wrong, just like Liddle’ Adam Schiff..”

Some of the president’s allies amplified the criticism on Sunday, with Miller, the senior White House adviser, calling the complaint a “partisan hit job.”

“This is a deep state operative, pure and simple,” he told host Chris Wallace on “Fox News Sunday.”

Even though the acting director of national intelligence, Joseph Maguire, has said he has no reason to doubt the sincerity of the whistleblower or the inspector general who investigated the complaint, Miller branded the anonymous complainant a “saboteur trying to undermine a democratically elected government.”

Sen. Lindsey Graham (R-S.C.), the chairman of the Senate Judiciary Committee and one of Trump’s strongest defenders in Congress, also cast doubt on the whistleblower and the entire Ukraine affair.

“This whole thing is a sham. … Who is this whistleblower?” Graham told host Margaret Brennan on CBS’s “Face the Nation.” “What bias do they have? Why did they pick this whistleblower to tell a hearsay story? The transcript does not match the complaint. This thing stinks.”

He also said he had “zero problems” with Trump’s July 25 phone conversation with the Ukrainian president, Volodymyr Zelensky.

Brennan asked what advice Graham gave the president when they golfed together on Saturday.

“Keep fighting back. We have your back on this,” Graham replied. “I am openly telling everybody in the country, I have the president’s back because I think this is a setup.”

Randy Lemmerman contributed to this report.