House Speaker Nancy Pelosi, D-Calif., said Sunday that President Donald Trump's conduct is "so much worse" than that of former President Richard Nixon, adding that Trump is insecure about being an "imposter."

Pelosi spoke with CBS's "Face the Nation" days after House impeachment investigators conducted their first public hearings. Three more days of public hearings are scheduled for this week.

"I will make sure he does not intimidate the whistleblower," Pelosi said of the CIA employee whose complaint about Trump's conduct toward Ukraine led to the impeachment inquiry. "The president can come before the committee and speak all the truth that he wants. ... He has every opportunity to make his case.

"But it's really a sad thing," Pelosi continued. "What the president did was so much worse than even what Richard Nixon did. At some point, Richard Nixon cared about the country enough to recognize that this could not continue."

Since the House launched its impeachment inquiry in September, multiple Trump administration officials have alleged that Trump tied U.S. aid to Ukraine to an investigation of former Vice President Joe Biden and his son Hunter Biden. Republicans have defended the president by pointing out that the aid was eventually released and that Ukraine never announced an investigation. But Pelosi and other Democrats have said that, by conditioning aid on the investigations, Trump was attempting to commit bribery.

"The whistle was blown, the whistle was blown, and that was blown long before we heard about it," Pelosi said. "Don't forget that in between all of that came the inspector general, an inspector general appointed by President Trump. And the inspector general said this was of urgent concern. That is what intervened."

Speaking with "Fox News Sunday," Rep. Jim Himes, D-Conn., said Republicans had offered up "a whole bunch of defenses that don't make sense."

"Lots of crimes can be committed ... by the boss hinting and giving direction," Himes said. "Corrupt people don't always say, 'Hey, here's the signed contract.' What has already developed from secondhand witnesses is that this aid was withheld as a condition."

Even more administration officials are set to testify publicly this week, including Gordon Sondland, the U.S. ambassador to the European Union, who Himes said was a firsthand witness to Trump's conduct, having been tasked with carrying out his wishes. While Sondland's initial testimony in October largely absolved him of any wrongdoing, he submitted additional testimony in November acknowledging that he did deliver a quid pro quo message to Ukraine.

Acknowledging that Sondland's credibility is in question, Himes said it "was not lost on Ambassador Sondland what happened to" Trump associates Roger Stone and Michael Cohen "for lying to Congress."

"My guess is Gordon Sondland is going to do his level best to tell the truth because, otherwise, he may have a very unpleasant legal future in front of him," said Himes, a member of the Intelligence Committee.

Democrats also addressed Trump's tweet bashing Marie Yovanovitch, the ousted U.Sa Ambassador to Ukraine, during her testimony on Friday — commentary that Yovanovitch said moments later was "intimidating" and that Democrats referred to as witness intimidation.

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Pelosi called Trump's tweet "totally wrong and inappropriate and typical of the president."

"People don't insult people, especially when they're giving testimony before the Congress of the United States," she said. "I think even his most ardent supporters have to honestly admit this was the wrong thing for the president to do.

"The president and perhaps some at the White House need to know is that the words of the president weigh a ton," she continued. "They are very significant. And he should not frivolously throw out insults. I think part of it is his own insecurity as an imposter. I think he knows full well he's in that office way over his head, and so he has to diminish everyone else."

Elsewhere on "Face the Nation," Rep. Mike Quigley, D-Ill., said the president's comment was "part of a pattern of witness intimidation."

"What did he say about Mr. Cohen? That he was a rat," Quigley, a member of the Intelligence Committee, said, adding that Trump "was talking like a mobster."

Referring to Paul Manafort, one of Trump's former campoaign managers, Quigley added: "What did he say about Mr. Manafort? That he was a good guy because he wasn't cooperating. That's witness intimidation."