Originally Published: 27 APR 23 17:06 ET

Updated: 27 APR 23 17:13 ET

By Katelyn Polantz, CNN Reporter, Crime and Justice

(CNN) -- Former Vice President Mike Pence testified on Thursday to a federal grand jury investigating the aftermath of the 2020 election and the actions of then-President Donald Trump and others, sources familiar with the matter told CNN.

The testimony marks a momentous juncture in the criminal investigation and the first time in modern history a vice president has been compelled to testify about the president he served beside.

Pence was poised to recount for the first time under oath his direct conversations with Trump leading up to January 6, 2021. Trump repeatedly pressured him unsuccessfully to block the 2020 election's result, including the morning of January 6 on a private phone call, and a federal judge previously ruled Pence could be compelled to recount conversations the two men had where Trump may have been acting corruptly.

Pence's meeting with investigators comes as he is exploring a possible challenge to Trump for the Republican presidential nomination in 2024, with his testimony likely to elicit a strong negative reaction from his former boss.

As part of his political appearances and a recent book tour, Pence frequently speaks about refusing to do Trump's bidding on January 6 and instead following the Constitution. But he had avoided speaking under oath as part of any investigation.

The grand jury in Washington, DC, whose proceedings are secret, assembled just before 9 a.m. ET on Thursday. That coincided with an increase in security inside the courthouse and two SUVs with tinted windows spotted ferrying people to the building.

Court battle

Special counsel Jack Smith's investigation around Trump's efforts to block the election result has long sought to question Pence under oath given his proximity to Trump at the White House.

Both Pence and Trump went to court to hold off his unprecedented subpoena. But trial and appellate judges ordered Pence to testify about his direct conversations with the then-President -- decisions that were in line with several other losses courts dealt to Trump as he's tried to block top officials from his administration from testifying.

The latest decision -- from the DC Circuit Court of Appeals, which refused to give Trump emergency help -- came Wednesday night.

The case has put Pence in a unique position to define the powers of his former office -- and the court had even given the former vice president the ability to keep out of criminal proceedings his actions while he served as the presiding officer of the Senate on January 6. Still, much of what Smith's team appears to be interested in would be attainable by the grand jury.

Pence and Trump's pre-January 6 communication

Trump's conversations with Pence and about Pence in the days before the US Capitol riot have been of keen interest to investigators probing the attack.

Though Pence declined to testify before the House January 6 committee that investigated the insurrection, people in Trump's orbit told the committee about a heated phone call he had with Pence the day of the attack in which he lobbed insults at his vice president. Pence and Trump did not speak during the attack on the Capitol itself, in which many of Trump's supporters angrily sought him out, and Pence narrowly escaped the mob heading to the Senate floor.

Much of what is known about Trump's communications with Pence leading up to the insurrection has come from a memoir the former vice president published last year, as well as from people who testified in the House probe into the attack.

Nicholas Luna, a former special assistant to Trump, said he remembered Trump calling Pence a "wimp." Luna said he recalled something to the effect of Trump saying, "I made the wrong decision four or five years ago."

And Julie Radford, Ivanka Trump's former chief of staff, said she recalled Ivanka Trump telling her that "her dad had just had an upsetting conversation with the vice president."

Radford said she was told that Trump had called Pence "the P-word," referencing a derogatory term.

For Pence's part, many of his public comments about his conversations with Trump in the days before and after the insurrection he disclosed in his memoir.

In the book, Pence wrote that Trump told him in the days before the attack that he would inspire the hatred of hundreds of thousands of people because he was "too honest" to attempt to overturn the results of the 2020 election.

The former vice president also said in the book that he asked his general counsel for a briefing on the procedures of the Electoral Count Act after Trump in a December 5 phone call "mentioned challenging the election results in the House of Representatives for the first time."

Over lunch on December 21, Pence wrote, he tried to steer Trump to listen to the White House counsel's team's advice, rather than outside lawyers, a suggestion the then-president shot down.

And Pence wrote that Trump told him in a New Year's Day phone call: "You're too honest," predicting that "hundreds of thousands are gonna hate your guts" and "people are gonna think you're stupid."

"Mr. President, I don't question there were irregularities and fraud," Pence wrote that he told Trump. "It's just a question of who decides, and under the law that is Congress."

This story has been updated with additional details.