CNN — Former Attorney General Bill Barr on Wednesday undermined a key pillar of his old boss’ defense in the special counsel’s probe into 2020 election interference, telling CNN’s Kaitlan Collins that Donald Trump “knew well he lost the election.”

The former attorney general also described Trump’s alleged actions as detailed in the indictment as “nauseating” and “despicable,” saying on “The Source,” “someone who engaged in that kind of bullying about a process that is fundamental to our system and to our self-government shouldn’t be anywhere near the Oval Office.”

A central premise of the special counsel’s case is that Trump knew the election claims he was making were false after being told by several close aides that he had lost the election. Trump’s lawyers have argued that his statements were protected under the First Amendment, a position that Barr, in his first interview since Trump’s third indictment, said was not a valid argument.

“As the indictment says, they are not attacking his First Amendment right. He can say whatever he wants, he can even lie. He can even tell people that the election was stolen when he knew better,” Barr said. “But that does not protect you from entering into a conspiracy.”

He added: “At first I wasn’t sure, but I have come to believe he knew well he had lost the election.”

Special counsel Jack Smith brought four charges against Trump on Tuesday – including conspiracy to defraud the United States – as part of his investigation into efforts to overturn the 2020 election leading up to the January 6, 2021, attack on the US Capitol. The former president is expected to be arraigned at the Washington, DC, federal courthouse Thursday.

While a loyalist for much of his tenure as Trump’s attorney general, Barr notably contradicted the former president’s claims that the election was rigged when he said in December 2020 that the Justice Department had not found substantial evidence of widespread voter fraud. He resigned soon after and has since emerged as one of Trump’s most significant critics.

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Watch: Bill Barr calls out Donald Trump’s election lies during January 6 testimony

Barr defended Smith in the interview Wednesday, breaking from many Republicans who have disparaged the special counsel’s probes into Trump as being politically motivated.

“He is the kind of prosecutor, in my view, that if he thinks someone has committed a crime, he, you know, homes in on it and really goes to try to make that case,” Barr said. “There’s no question he’s aggressive but I do not think he’s a partisan actor.”

The former attorney general added that he believes Smith has more evidence to prove that the former president knew the election was not stolen.

“We’re only seeing the tip of the iceberg on this,” Barr said. “I think there is a lot more to come, and I think they have a lot more evidence as to President Trump’s state of mind.”

Barr declined to say whether he has spoken with Smith’s team as part of the investigation. The former attorney general spoke with the House select committee that investigated January 6 last year and described Trump as “detached from reality” after the 2020 election.

As Trump’s legal trouble looms, the former president is running to return to the White House and currently leads the GOP field. While Barr said his former boss should not be the Republican presidential nominee in 2024, he would not on Wednesday rule out voting for Trump if he were to be the nominee.

“I will have to wait to see what the situation is and I will pick my poison at that point,” Barr said.

CNN’s Sam Fossum contributed to this report.