Ex-Trump adviser: The best way to self-impeach is to hire Rudy Giuliani

"It's not only a conspiracy theory, it is completely debunked. I don't want to be glib about this matter, but last year, retired former Sen. Judd Gregg wrote a piece in The Hill magazine saying the 3 ways or the 5 ways to impeach one's self. And the 3rd way was to hire Rudy Giuliani. And at this point, I am deeply frustrated with what he and the legal team is 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 again repeat that it has no validity."

Why it matters: In his phone call with Ukrainian President Volodymyr Zelensky, Trump claimed that the hacked DNC server is in Ukraine and asked Zelensky to work with Attorney General Bill Barr to "get to the bottom of it." The assertion is part of an easily debunked right-wing conspiracy theory that alleges that CrowdStrike, the first firm to publicly release evidence that Russia perpetrated the hack, made up information to fuel the Russia investigation.

The big picture: Bossert said that while he doesn't believe Trump was "pressuring" the Ukrainian president, he was "deeply disturbed" by the phone call and said that Trump could be in serious trouble as House Democrats' formal impeachment inquiry heats up: "It is a bad day and a bad week for the president and for this country if he is asking for political dirt on an opponent."

• Bossert added that Trump needs to "move forward" from 2016 collusion allegations and that Giuliani's obsession with the Ukraine-DNC conspiracy could help bring down the president.

Go deeper: Trump, Ukraine's president and the call heard 'round the world