WASHINGTON — John R. Bolton, the former White House national security adviser, said on Monday that he was willing to testify at President Trump’s impeachment trial if he was subpoenaed.
“I have concluded that, if the Senate issues a subpoena for my testimony, I am prepared to testify,” Mr. Bolton said in a statement on his website.
The development is a dramatic turn in the impeachment proceeding, which has been stalled over Democrats’ insistence on hearing from critical witnesses Mr. Trump blocked from testifying in the House inquiry into his pressure campaign on Ukraine. Mr. Bolton is a potential bombshell of a witness, with crucial knowledge of the president’s actions and conversations regarding Ukraine that could fill out key blanks in the narrative of the impeachment case.
His willingness to tell the Senate what he knows ratchets up pressure on Senator Mitch McConnell, Republican of Kentucky, who has refused to commit to calling witnesses at the impeachment trial, to change his stance. It is unclear how the White House will respond to Mr. Bolton’s declaration, but his statement strongly suggested that he would testify regardless of whether Mr. Trump sought to prevent him.
A spokesman for Mr. McConnell declined to comment Monday afternoon shortly after the announcement.
“It now falls to the Senate to fulfill its constitutional obligation to try impeachments, and it does not appear possible that a final judicial resolution of the still-unanswered constitutional questions can be obtained before the Senate acts,” Mr. Bolton wrote. “Accordingly, since my testimony is once again at issue, I have had to resolve the serious competing issues as best I could, based on careful consideration and study.”
If he did appear under oath in the Senate, Mr. Bolton would be the closest adviser to the president to testify about what Mr. Trump said behind closed doors as he pressured the Ukranians to investigate his political rivals as he was withholding nearly $400 million in military aid from the country.
The Democratic-led House impeached Mr. Trump last month on charges of abuse of power and obstruction of Congress, charging him with a corrupt scheme to solicit help from Ukraine in the 2020 election, and concealing his actions from Congress.
Mr. Trump has repeatedly sought to block his most senior aides, as well as former advisers who have left the White House, from speaking to Congress, and has gone to court to stop several of them from cooperating.
Mr. Bolton declined to say on Monday precisely what he would be willing to tell Congress. But former White House officials and people close to Mr. Bolton have indicated that his testimony would likely be damning to Mr. Trump and put additional pressure on moderate Republicans to consider convicting him.
That could fundamentally change the dynamics around the impeachment trial in the Senate, where a two-thirds vote — 67 senators — is needed to remove Mr. Trump. Democrats, the minority party, control 45 seats.
Speaker Nancy Pelosi has so far declined to send the Senate the charges against Mr. Trump, which would trigger the start of the trial, saying that she wants assurances that Mr. McConnell will run a fair process.
Although Mr. Bolton never spoke with House investigators, his aides provided them with a portrait of how he viewed Mr. Trump’s dealings with Ukraine. The aides said that Mr. Bolton was deeply concerned about how Mr. Trump and his personal lawyer, Rudolph W. Giuliani, pressured the Ukranians to investigate Democrats. Other officials testified under oath that Mr. Bolton told White House colleagues that Mr. Giuliani was a “hand grenade who’s going to blow everybody up.”
Late last year, the chances of Mr. Bolton testifying looked bleak. In October, the House subpoenaed Mr. Bolton’s deputy, Charles Kupperman, but the White House tried to block him from testifying. Mr. Kupperman’s lawyer, Charles Cooper, who also represents Mr. Bolton, filed a lawsuit asking a federal judge to decide on what Mr. Kupperman should do. The House withdrew the subpoena, as leading Democrats argued it was not worth awaiting the outcome of a lengthy — potentially yearslong — legal proceeding before moving to impeach Mr. Trump.
The judge ruled late last month that the issue was moot, leaving the question of whether the president’s closest advisers had to testify unresolved.