(CNN) — A federal judge has decided that former Vice President Mike Pence must testify to a grand jury about conversations he had with Donald Trump leading up to January 6, 2021, according to multiple sources familiar with a recent federal court ruling.

But the judge said — in a ruling that remains under seal — that Pence can still decline to answer questions related to his actions on January 6 itself, when he was serving as president of the Senate for the certification of the 2020 presidential election, according to one of the sources.

The ruling from chief judge James Boasberg of the US District Court in Washington, DC, is a major win for special counsel Jack Smith, who is spearheading the Justice Department investigation. Pence still has the ability to appeal.

The former vice president said his team is “evaluating the court’s decision,” telling Newsmax’s Greta Van Susteren in an interview Tuesday that “the requirements of my testimony going forward are a subject of our review right now, and I’ll have more to say about that in the days ahead.”

In the lead-up to the congressional certification vote, Pence faced enormous pressure from Trump and his allies to disrupt lawmakers’ plans to validate Joe Biden’s win. As president of the Senate, Pence was tasked with presiding over the certification proceedings.

Trump’s conversations with Pence in the days surrounding the insurrection 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.

Nicholas Luna, a former special assistant to Trump, told the committee 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 have come in a memoir he published last year.

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 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.”

Big win for special counsel

Smith is investigating the Trump-aligned effort to subvert the 2020 election. Smith subpoenaed Pence for testimony and documents earlier this year.

Days after news broke of the subpoena, Pence and his advisers indicated that the former vice president would challenge the subpoena under the Constitution’s Speech or Debate Clause, which shields lawmakers from certain law enforcement actions connected to their legislative duties.

“I am going to fight the Biden DOJ subpoena for me to appear before the grand jury because I believe it’s unconstitutional and unprecedented,” Pence said at an event in February. He has suggested that — because he was also serving as president of the Senate during the January 6 certification vote — the constitutional clause covered the conduct that investigators are looking at.

The challenge in court has played out in secret.

Pence’s claims, as he has described them publicly, are seen as novel. His arguments attracted criticism from a broad range of legal scholars, including former Judge Michael Luttig, a conservative legal luminary who publicly argued that Pence should certify the electoral results.

Even as Pence has fought the subpoena, he has stood by his refusal to disrupt the congressional certification of Biden’s win, as Trump called upon him to do.

This story has been updated with additional details.

Mike Pence must testify about conversations he had with Donald Trump leading up to January 6, judge rules