John Bolton, former national security adviser to President Donald Trump, said Monday that he is prepared to testify to the Senate if subpoenaed for the impeachment trial.

“I have concluded that, if the Senate issues a subpoena for my testimony, I am prepared to testify,” Bolton said in a statement sent to reporters and posted on his political action committee website.

Bolton was serving in the White House during the decision to temporarily halt congressional aid delivered to Ukraine.

He left the administration in September 2019, just before the effort became public and Democrats launched the impeachment effort.

In November 2019, Bolton refused to testify to the House Intelligence Committee during the impeachment investigation. He noted in his statement that the House had concluded its investigation.

“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 said.

Republican-led Senate members have indicated they are not interested in pursuing additional evidence or calling more witnesses to testify in the trial. But the trial will not begin unless House Speaker Nancy Pelosi sends the articles of impeachment to the Senate, a step she stalled prior to the Christmas break.