Former Vice President Mike Pence appeared today before the grand jury hearing evidence about former President Donald Trump’s efforts to cling to power after he lost the 2020 election, a person briefed on the matter said.

Pence’s appearance behind closed doors at the U.S. District Court in Washington came after he was subpoenaed to testify before the grand jury earlier this year. Pence is considered a key witness, given the pressure campaign that Trump engaged in to try to convince him to play a critical role in blocking or delaying congressional certification of Joe Biden’s victory.

Pence, now a potential rival to Trump for the 2024 Republican presidential nomination, rebuffed Trump’s demands in January 2021 that he use his ceremonial role overseeing the certification of the Electoral College results to derail the final step in affirming Biden’s victory.

Pence’s advisers had discussions with Justice Department officials last year about providing testimony in their criminal investigation into whether Trump and a number of his allies broke federal law in trying to keep Trump in power. But the talks broke down, leading prosecutors to seek a subpoena for Pence’s testimony.

Both Pence and Trump tried to fight the subpoena, with the former vice president claiming it violated the “speech or debate” clause of the Constitution given his role as president of the Senate on Jan. 6, 2021, and Trump claiming their discussions were covered by executive privilege.

Trump’s efforts to prevent testimony based on executive privilege claims were rebuffed by the courts. Pence partially won in his effort to forestall or limit his testimony; the chief judge overseeing the grand jury ruled that he would not have to discuss matters connected to his role as president of the Senate on Jan. 6, but that he would have to testify to any potential criminality by Trump.

Pence’s unwillingness to go along with Trump’s plan to block or delay certification of the electoral outcome infuriated Trump, who assailed his vice president privately and publicly on Jan. 6.

Pence subsequently became a target of the pro-Trump mob that swamped the Capitol building that day, with some chanting “Hang Mike Pence!” as they moved through the complex. Someone brought a fake gallows that stood outside the building.

It is not clear what testimony Pence provided today. But prosecutors are surely interested in Pence’s accounts of his interactions with Trump and Trump advisers including John Eastman, a lawyer who promoted the idea that they could use the congressional certification process on Jan. 6 to give Trump a chance to remain in office.

That plan relied on Pence using his role as president of the Senate to hold up the process. But Pence’s top lawyer and outside advisers concluded that the vice president did not have the legal authority to do so.

Pence described some of his conversations with Trump in his memoir, “So Help Me God.”

Pence described in the book how Trump worked with Eastman to pressure him into doing something that the vice president was clear that he could not and would not do. He wrote that on the morning of Jan. 6, Trump used a phone to try to bludgeon him again.

“You’ll go down as a wimp,” the president told the vice president. “If you do that, I made a big mistake five years ago!”

Some of Pence’s aides have already appeared before the grand jury, which is investigating various matters related to the attack on the Capitol, efforts to use the levers of government to keep Trump in power, and a plan for so-called “fake” Trump electors to be available if there were objections in Congress on Jan. 6 to the results in a number of swing states won by Biden.

This article originally appeared in The New York Times.