Former Vice President Mike Pence appeared Thursday before a federal grand jury investigating former President Trump’s efforts to remain in power after the 2020 election, a source familiar with the matter confirmed to The Hill.

His testimony is a major development in special counsel Jack Smith’s investigation into Trump’s conduct around the 2020 election and the Jan. 6, 2021, attack on the Capitol, when rioters attempted to stop Pence and lawmakers from certifying President Biden’s victory.

The former vice president reportedly spent several hours testifying at a courthouse in Washington, D.C.

Pence has maintained he has nothing to hide from investigators about the fallout after the 2020 election.

Pence had previously challenged a subpoena from Smith for his testimony in the case on the grounds that serving as the former president of the Senate essentially made him a member of the legislative branch on the day of the riots, and he would therefore be shielded from the subpoena under the “speech and debate” clause of the Constitution.

A federal judge earlier this month ruled that Pence had to provide testimony about Trump’s efforts to overturn the 2020 election results, though the judge said Pence would not be compelled to testify about his role in Congress.

The former vice president’s testimony will likely give the grand jury additional details about Trump’s conversations and behavior in the time between the November 2020 election and Jan. 6. Trump spent that time repeatedly insisting the election was fraudulent and suggesting Pence had the unilateral power to reject the Electoral College results.

The former vice president in his memoir detailed some of the conversations he had with Trump in the weeks leading up to that day, writing about how the former president repeatedly pressured him to consider rejecting the 2020 election results. Pence ultimately certified the results, saying that the constitution did not give him the unilateral power to overturn the will of the people.

“He was pretty explicit in his book and wrote about it pretty extensively, so there really shouldn’t be any secrets there for people anxious to know what it is he would testify,” Pence adviser Marc Short said Thursday on NewsNation’s “The Hill.”

Since leaving office, Pence has been critical of Trump over the events of Jan. 6, saying the two will likely never see eye-to-eye about that day.

Pence is also mulling a 2024 White House bid, which would put him on a collision course with Trump, who is already a declared candidate and leading national polls of GOP primary voters by a wide margin. Pence has repeatedly said he believes the Republican Party will have better options than his old running mate, and he has signaled he will make a decision about his own plans for 2024 by June.

Updated 6:09 p.m.