WASHINGTON — Former national security adviser John Bolton says he is now willing to testify in the Senate impeachment trial if subpoenaed.
In a statement posted Monday, Bolton writes, "I have concluded that, if the Senate issues a subpoena for my testimony, I am prepared to testify."
Bolton had a front-row seat to the White House’s pressure campaign against Ukraine to investigate the son of Trump’s political rival, Joe Biden, including the decision to withhold military aid from Ukraine. He served as Trump’s national security adviser for more than a year, until his departure in September just a couple of weeks before the Ukraine pressure effort became public.
White House National Security Advisor John Bolton
White House National Security Advisor John Bolton
John Bolton, national security advisor, speaks during a White House press briefing in Washington, D.C., U.S., on Thursday, Aug. 2, 2018. Russian efforts to interfere in upcoming U.S. midterm elections have yet to reach the intensity of the Kremlin's campaign to disrupt the 2016 presidential vote, but they're only 'a keyboard click away' from a more serious attack, Director of National Intelligenceï¿½Dan Coatsï¿½said. Photographer: Zach Gibson/Bloomberg via Getty Images
OXON HILL, MD, UNITED STATES - 2018/02/22: John Bolton, Former United States Ambassador to the United Nations, at the Conservative Political Action Conference (CPAC) sponsored by the American Conservative Union held at the Gaylord National Resort & Convention Center in Oxon Hill. (Photo by Michael Brochstein/SOPA Images/LightRocket via Getty Images)
WASHINGTON, DC - NOVEMBER 08: US Ambassador to United Nations John Bolton speaks at the National Oversight and Government Reform Committee on moving the U.S. Embassy in Israel to Jerusalem on Capitol Hill on November 8, 2017 in Washington, DC. (Photo by Tasos Katopodis/Getty Images)
Former US Ambassador to the United Nations John Bolton speaks during the American Conservative Union Conference March 6, 2014 in National Harbor, Maryland. The annual conference is a meeting of politically conservatives Americans. AFP PHOTO/Brendan SMIALOWSKI (Photo credit should read BRENDAN SMIALOWSKI/AFP/Getty Images)
NASHUA, NH - APRIL 17: Former Ambassador to the United Nations John Bolton speaks at the First in the Nation Republican Leadership Summit April 17, 2015 in Nashua, New Hampshire. The Summit brought together local and national Republicans and was attended by all the Republicans candidates as well as those eyeing a run for the nomination. (Photo by Darren McCollester/Getty Images)
LAS VEGAS, NV - MARCH 29: Former United States ambassador to the United Nations John Bolton speaks during the Republican Jewish Coalition spring leadership meeting at The Venetian Las Vegas on March 29, 2014 in Las Vegas, Nevada. The Republican Jewish Coalition began its annual meeting with potential Republican presidential candidates in attendance, along with Republican super donor Sheldon Adelson. (Photo by Ethan Miller/Getty Images)
DES MOINES, IA - JANUARY 24: Former Ambassador to the United Nations John Bolton speaks to guests at the Iowa Freedom Summit on January 24, 2015 in Des Moines, Iowa. The summit is hosting a group of potential 2016 Republican presidential candidates to discuss core conservative principles ahead of the January 2016 Iowa Caucuses. (Photo by Scott Olson/Getty Images)
WASHINGTON - DECEMBER 4: US President George W. Bush (R) and Ambassador to the UN John Bolton (L) meet in the Oval Office of the White House December 4, 2006 in Washington, DC. Bush accepted Bolton's resignation as Ambassador to the United Nations when his term is up in January 2007. (Photo by Brendan Smialowski/Getty Images)
HOUSTON, TX - MAY 03: Former U.S. ambassador to the United Nations John Bolton speaks during the 2013 NRA Annual Meeting and Exhibits at the George R. Brown Convention Center on May 3, 2013 in Houston, Texas. More than 70,000 peope are expected to attend the NRA's 3-day annual meeting that features nearly 550 exhibitors, gun trade show and a political rally. The Show runs from May 3-5. (Photo by Justin Sullivan/Getty Images)
John Bolton, former US ambassador to the United Nations (R) and Aaron Abramovitch, Director-General of Israeli Ministry of Foreign Affairs attend a panel during the eighth annual Herzliya Conference in Herzliya, 22 January 2008. The eight annual Herzliya Conference, entitled Balance of Israel's National Security, and coordinated by the Institute for Policy and Strategy at the Interdisciplinary Center Herzliya's Lauder School of Government, started yesterday and lasts for 3 days. The theme for this year's conference is 'Israel at 60: Tests of Endurance.' AFP PHOTO/JACK GUEZ (Photo credit should read JACK GUEZ/AFP/Getty Images)
NEW YORK - OCTOBER 14: U.S. Ambassador to the United Nations John Bolton addresses the Security Council after it unanimously voted in favor of the resolution for sanctions against North Korea at the United Nations headquarters October 14, 2006 in New York City. The council voted unanimously to approve the resolution which demands that North Korea destroy all of its nuclear weapons and bans the import and export of materials used for the creation of weapons of mass destruction. (Photo by Stephen Chernin/Getty Images)
(FILES) A file picture dated 10 October 2006 shows former US ambassador to the United Nations John Bolton speaking to the media after a meeting of the five permanent members of the Security Council (Britain, China, France, Russia and United States) plus Japan, where they discussed a resolution on the North Korea nuclear situation at the UN headquarters in New York. Bolton said 21 January 2008 that Israel may have to take military action to prevent its archfoe Iran from acquiring an atomic bomb. Bolton also said that further UN sanctions against the Islamic republic will be ineffective in stopping Iran's controversial nuclear programme which Israel and the US believe is aimed at developing a bomb -- a claim denied by Tehran. AFP PHOTO/Stan HONDA (Photo credit should read STAN HONDA/AFP/Getty Images)
UNITED NATIONS, UNITED NATIONS: John Bolton (C), United States Ambassador to the United Nations, speaks to the media 13 October 2006 before a Security Council meeting about Georgia, to be followed by discussions on the North Korea resolution at UN headquarters in New York. The UN Security Council on Friday was set to consider a compromise draft resolution mandating wide-ranging sanctions against North Korea over its declared nuclear test but specifically ruling out the use of force. AFP PHOTO/Stan HONDA (Photo credit should read STAN HONDA/AFP/Getty Images)
NEW YORK - NOVEMBER 13: John Bolton, U.S. ambassador to the United Nations, speaks to the media after a meeting with the five permanent members of the Security Council and Germany concerning Iran at the French Mission to the UN November 13, 2006 in New York City. Bolton received a controversial recess appointment to the post by President Bush in August 2005 and was renominated last week, but would face confirmation from a new Democrat-controlled Senate if not voted on by the current Congress' recess in January. Democrats oppose the nomination. (Photo by Michael Nagle/Getty Images)
White House National Security Advisor John Bolton, U.S. Director of National Intelligence Dan Coats, FBI Director Christopher Wray, arrive to attend a briefing on election security in the White House press briefing room at the White House in Washington, U.S., August 2, 2018. REUTERS/Carlos Barria
U.S. National Security Adviser John Bolton attends a news conference in Moscow, Russia June 27, 2018. REUTERS/Sergei Karpukhin
HELSINKI, FINLAND JULY 16, 2018: US Ambassador to Russia Jon Huntsman Jr (C) and US National Security Adviser John Bolton (R) talking as Russian President Vladimir Putin and US President Donald Trump meet at the Presidential Palace. Alexei Nikolsky/Russian Presidential Press and Information Office/TASS (Photo by Alexei Nikolsky\TASS via Getty Images)
WASHINGTON, DC - AUGUST 02: National Security Advisor John Bolton, briefs the media on election interference, at the White House, on August 2, 2018 in Washington, DC. The administration's top security officials briefed the media on election interference. (Photo by Mark Wilson/Getty Images)
National Security Advisor John Bolton (R) attends a joint press conference of the US and Russian Presidents after a meeting at the Presidential Palace in Helsinki, on July 16, 2018. - The US and Russian leaders opened an historic summit in Helsinki, with Donald Trump promising an 'extraordinary relationship' and Vladimir Putin saying it was high time to thrash out disputes around the world. (Photo by Brendan SMIALOWSKI / AFP) (Photo credit should read BRENDAN SMIALOWSKI/AFP/Getty Images)
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Bolton has previously said he would not testify before the House Intelligence Committee during its impeachment investigation unless he was subpoenaed and a judge ordered him to defy the White House by appearing before Congress.
But he said in his statement that "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, and it does not appear possible that a final judicial resolution of the still-unanswered Constitutional questions can be obtained before the Senate acts."
The House has yet to transmit the two articles of impeachment passed at the end of December to the Senate, where leaders remain deadlocked over the parameters of how the trial would be conducted. One of the primary divisions is over Democratic calls to include new witnesses, such as Bolton.