Former National Security Adviser John Bolton announced Monday that he is “prepared to testify” if the Senate issues him a subpoena for President Trump’s impeachment trial. “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,” he said in a statement on his website. “I have concluded that, if the Senate issues a subpoena for my testimony, I am prepared to testify.” Bolton’s announcement is a major shift in the impeachment proceeding as Trump has repeatedly tried to block most senior aides, along with former advisers, from testifying in the House inquiry into his pressure campaign on Ukraine. Bolton would be the closest adviser to Trump with key knowledge about closed-door conversations. He is a crucial witness for the case against the president, as his lawyers have previously said he is knowledgeable about “many relevant meetings and conversations” connected to the issues at the heart of the impeachment inquiry.

“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,” Bolton wrote. His willingness to cooperate with a Senate subpoena is likely to ramp up pressure on Sen. Mitch McConnell (R-KY), who has consistently refused to commit to calling witnesses to testify at the trial.