House Speaker Nancy Pelosi says that President Trump's ongoing attacks on witnesses in the impeachment probe — including his tweets about former U.S. Ambassador to Ukraine Marie Yovanovitch Friday — are "very significant" as the impeachment probe progresses.

"Why do you think he was tweeting about her?" Brennan asked Pelosi, in reference to Mr. Trump's tweets during Yovanovitch's testimony Friday.

"Because he knows — well, he made a mistake and he knows her strength. And he was trying to undermine it," Pelosi replied.

She added, "Of course, presidents appoint ambassadors, but people don't insult people, especially when they're giving testimony before the Congress of the United States. I think even his most ardent supporters have to honestly admit this is the wrong thing for the president to do."

"Everywhere Marie Yovanovitch went turned bad," the president tweeted about Yovanovitch's past diplomatic posts in the middle of her testimony on Friday. "She started off in Somalia, how did that go? Then fast forward to Ukraine, where the new Ukrainian President spoke unfavorably about her in my second phone call with him."

Some Democrats interpreted this as witness tampering or an attempt to intimidate Yovanovitch. The White House maintained that it's just his opinion and denied the president was trying to intimidate her. Pelosi said the White House "has to know that the words of the president weigh a ton."

"He should not frivolously throw out insults, but that's what he does. I think part of it is his own insecurity as an imposter. I think he knows full well that he's in that office way over his head. And so he has to diminish everyone else," she added.

In her public testimony on Friday, Yovanovitch testified about a "smear campaign" led by Trump personal attorney Rudy Giuliani, which resulted in her ouster. She surmises that the attacks against her stem from her anti-corruption work in Ukraine. The diplomat lamented before the House Intelligence Committee about how "foreign corrupt interest could manipulate our government."

For more of Margaret Brennan's interview with Speaker Pelosi tune in this Sunday to "Face the Nation" at 10:30 a.m. EST