John Bolton, former national security advisor to Donald Trump, issued a statement Monday that he is prepared to testify if he is subpoenaed by the Senate in its impeachment trial of the president.

“The House has concluded its Constitutional responsibility by adopting Articles of Impeachment related to the Ukraine matter. It now falls to the Senate to fulfill its Constitutional obligation to try impeachments,” Bolton wrote.

Bolton was not subpoenaed during the House of Representatives’ impeachment inquiry but resolved to testify pending the outcome of a question before federal court concerning his colleague Charles Kupperman. Kupperman, like Bolton, claimed he was receiving conflicting demands from the Executive and Legislative branches of the government and sought a declaratory judgment from the court as to whether he needed to comply with the subpoena. This question was never resolved as the House withdrew its subpoena from Kupperman.

“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 Monday. “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.”