President Donald Trump's former homeland security adviser, Tom Bossert, expressed strong concerns Sunday about the president's July phone call with Ukrainian President Volodymyr Zelenskiy and said if Trump does not give up his fixation on debunked 2016 election allegations, it will "bring him down."

"I’m deeply disturbed by it [the call] as well and this entire mess has me frustrated," Bossert said on ABC's "This Week" when asked about a whistleblower complaint made public last week. "I’ve just spent a week overseas, and I’ll tell you, the whole world is watching this. The removal of a president is a — is a big and serious deal. But the removal of a president in not only a democracy, but the biggest democracy in the world is really a weighty matter, and I hope that everyone can sift through the evidence and be very careful, as I’ve seen a lot of rush to judgment this week."

"That said, 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," he continued. "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."

Bossert's comments come amid a series of revelations surrounding the president's Ukrainian efforts, namely the Thursday release of a whistleblower's complaint at the center of the ordeal. The whistleblower wrote that White House officials were so concerned about what Trump said in a July call with Zelenskiy that they intervened to "lock down" the record of the conversation.

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The whistleblower, whose name and gender have not been released, lodged the formal complaint out of a belief that Trump was "using the power of his office to solicit interference from a foreign country" in the 2020 election, the complaint says.

In the call, Trump discussed Zelenskiy help investigate the Biden family's business dealings. Former Vice President Joe Biden's son, Hunter, was on the board of a Ukrainian gas company. The matter is now the subject of a formal impeachment inquiry launched by the House this week.

The whistleblower also wrote of learning about a "sudden change of policy with respect to U.S. assistance for Ukraine" in mid-July that executive branch officials could not explain. The Trump administration froze nearly $400 million in aid to Ukraine, releasing that hold just before Democrats in Congress revealed the existence of the whistleblower complaint.

On Wednesday, the White House released a detailed summary of the president's discussion with Zelenskiy, which showed Trump asked the Ukrainian president to look into why that country's top prosecutor apparently had ended an investigation into the company, Burisma Holdings, where Biden’s son was a board member.

Bossert, who was fired from the administration last year shortly after now ex-national security adviser John Bolton took that post, said he "didn't see" Trump pressuring Zelenskiy in the call but understood why it was being interpreted in that way.

"I’ve spent a lot of time with this president, and I can easily see other reasons for why this president might have delayed the aide to Ukraine and those Javelin missiles," he said. "In fact, as you know, President Obama considered this deeply and decided not to provide lethal military support. President Trump and I and others spent quite a bit of time talking about this."

"So why he delayed is the key question," Bossert continued.

At another point in the call, Trump appeared to ask Zelenskiy about the debunked "CrowdStrike" conspiracy regarding the hack of the Democratic National Committee's emails. That theory turns the focus away from Russia's culpability — which former special counsel Robert Mueller said was the case in his extensive report — and instead place some doubt into whether Russia was the culprit.

"It’s not only a conspiracy, it is completely debunked," Bossert said, expressing frustration with Trump's personal attorney Rudy Giuliani, who's played an extensive role in the Ukraine gambit, adding, "And at this point 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 ... let me just again repeat that it has no validity.

"United States government reached its conclusion on attributing to Russia the DNC hack in 2016 before it even communicated it to the FBI, long before the FBI ever knocked on the door at the DNC, he continued. "So a server inside the DNC was not relevant to our determination to the attribution. It was made up front and beforehand."

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"The DNC server and that conspiracy theory has got to go, they have to stop with that, it cannot continue to be repeated in our — in our discourse," Bossert added.

Bossert closed by warning Trump to quit focusing on allegations from the 2016 election, though he understood the president's interest in them.

"If he continues to focus on that white whale, it’s going to bring him down," Bossert said. "Enough. The investigation’s over, there was no evidence of collusion. He’s won and he should take that victory and move forward."

Appearing on the show after Bossert, Giuliani denied talking about CrowdStrike with Trump and said he did not doubt Russia's involvement in the hacking.