In a surprise move, former national security adviser John Bolton on Monday said he would be willing to testify in the Senate’s impeachment trial of President Donald Trump if he is subpoenaed.

“Based on careful consideration and study,” Bolton said in a statement that was emailed to reporters on Monday with the subject line ‘URGENT,’ “I have concluded that, if the Senate issues a subpoena for my testimony, I am prepared to testify.”

The sudden reversal — Bolton previously said he would not appear before Congress unless a judge ordered him to defy the White House’s orders — puts fresh pressure on Senate Republicans to allow new witnesses at the impeachment trial. And unlike some of the witnesses who testified in the House impeachment inquiry, who were berated as low-level career officials repeating hearsay, Bolton would be difficult to dismiss.

He had a front-row seat to many of the key events that became the focus of the inquiry into whether Trump abused the power of the presidency by pressuring Ukraine to open investigations into his political rivals. He would be the closest and most senior advisor to Trump to testify, with the potential to reveal damaging information about what went on behind closed doors.

However, four Republicans would have to join Democrats to issue a subpoena for his testimony, a scenario many on Capitol Hill on Monday dismissed as highly unlikely. “Absolutely not. There’s a better chance he shaves his mustache,” a Senate Republican aide told TIME about the possibility of Bolton testifying at the impeachment trial.

When asked how the White House was reacting to Bolton’s offer to testify, one White House official joked, “Bolton who?” Another White House official declined to comment on how Trump is reacting to Bolton’s statement.

Bolton’s offer on Monday reversed his previous position and seemed to come as a surprise to Senate Republicans. Trump repeatedly tried to block both current and former White House aides from cooperating in the impeachment inquiry, even going to court to stop them. While many of his colleagues defied the White House in order to testify anyway, Bolton pre-empted Democrats by making it clear that he would not do so unless a court compelled him to. But he was coy in doing so, with his lawyer revealing in November that he had been part of “many relevant meetings and conversations that have not yet been discussed.”

While Bolton became active on Twitter after abruptly leaving his post as Trump’s third national security adviser in September, posting about foreign policy matters from North Korea to Iran, he has not publicly discussed what he witnessed at the White House related to the Ukraine matter.

Democrats, who have said Bolton’s testimony is crucial to the impeachment inquiry, quickly seized on the unexpected statement to press for witnesses at the trial. “Momentum for uncovering the truth in a Senate trial continues,” Senate Minority Leader Chuck Schumer said in a statement on Monday. “It is now up to four Senate Republicans to support bringing in Mr. Bolton, and the other three witnesses…to ensure all the evidence is presented at the onset of a Senate trial.”

The House voted along party lines to impeach Trump on Dec. 19, but Speaker Nancy Pelosi has yet to send the two articles of impeachment – for abuse of power, and for obstruction of Congress – over to the Senate. She has said that she is waiting for McConnell and Schumer to agree on “fair” rules for the trial. It was unclear on Monday how Bolton’s offer might play into these ground rules.

Last month, McConnell dismissed Democrats’ calls for Bolton’s testimony. “If the House plows ahead, if this ends up here in the Senate, we certainly do not need jurors to start brainstorming witness lists for the prosecution and demanding to lock them in before we’ve even heard opening arguments,” McConnell said on Dec. 17.

Indeed, Bolton’s new willingness to appear before lawmakers does not on its own upend the political reality on the Hill. While Senate Democrats are still campaigning for witnesses, Senate Republicans remain mostly unified behind McConnell, who says his chamber’s role is to judge the evidence brought forward by the House — not to dig up new evidence to address the House’s charges. Republicans could still be able to dictate the terms of the trial along a party-line vote. This suggests that Bolton’s offer, while titillating, could be moot unless perceived swing votes like Susan Collins of Maine and Lisa Murkowski of Alaska decide to buck McConnell and invite testimony.

If Democrats can pull that off and Bolton does testify, he could provide crucial information. Some of Bolton’s colleagues on the National Security Council, most notably Fiona Hill, who served as Trump’s top Russia expert, testified in detail about how he reacted to the Ukraine pressure campaign. During a White House meeting with Ukrainian officials on July 10, he became visibly furious when the U.S. Ambassador to the E.U. Gordon Sondland insinuated a quid pro quo, telling Ukrainian officials they would get a coveted White House meeting between the two presidents if they opened the probes south by Trump, according to Hill. He urged her to report what had transpired to the White House lawyers, calling the Ukraine scenario a “drug deal” and the president’s personal attorney Rudy Giuliani a “hand grenade who’s going to blow everybody up.”

— With reporting from Alana Abramson, Brian Bennett and Philip Elliott / Washington

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