Veteran journalist Bob Woodward said it became evident that "a line clearly was crossed" when Sen. Kamala Harris, D-Calif., got Attorney General William Barr to stammer during a Senate Judiciary Committee hearing on Wednesday.

Harris, a 2020 candidate for president, asked Barr if President Trump or anyone else at the White House suggested to him that he open an investigation into anyone. Barr struggled to answer, saying at one point that he was "trying to grapple with the word 'suggest.'"

[Watch: Barr testifies about Mueller investigation before Senate Judiciary Committee]

During an interview Thursday on MSNBC's "Morning Joe," Woodward said he was "incredulous" by what he saw.

He declined the notion that Barr was acting as Trump's "defense attorney," but rather said "it's that something happened."

"A line clearly was crossed because he didn't say no," Woodward said. Noting that Barr is "a man who traditionally has had a lot of spine," he added that Barr should have told whoever it was at the White House that such a suggestion was "inappropriate."

Woodward also complimented Harris, a former California attorney general, for her line of questioning.

"That's one of the great moments of interrogation by Sen. Harris because the body language, the hesitation, the telegraphing, you know you're asking me about something I really don't want to talk about," Woodward said.

Harris also got Barr to admit that he decided not to charge Trump with obstruction of justice without seeing the underlying evidence of special counsel Robert Mueller's Russia investigation report.

After leaving the room, Harris told reporters that Barr "lacks all credibility" and called on him to resign.