Trump impeachment: Former national security adviser John Bolton says he will testify if subpoenaed

Ambassador John Bolton, Donald Trump's ex-national security adviser and a central figure in the impeachment case against his former boss, has said he will testify before the Senate if it votes to subpoena him.

"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. I have concluded that, if the Senate issues a subpoena for my testimony, I am prepared to testify," Mr Bolton said in a statement posted to the website of his political action committee.

Mr Bolton had pledged to abide by the result of a court case brought by his former deputy, Dr Charles Kupperman. Dr Kupperman sought to have a federal judge determine whether he would be required to testify in the House of Representatives impeachment inquiry into whether Mr Trump violated his oath of office by withholding $391 million in military aid to Ukraine, in order to force that country's president to announce investigations into former Vice President Joe Biden and a debunked conspiracy theory which posits that Ukraine – not Russia – interfered in the 2016 presidential election.

But the former White House aide noted that the judge, Richard Leon, had "in a carefully reasoned opinion", found Dr Kupperman’s case to be moot because the House had completed its inquiry, "and therefore did not reach the separation-of-powers issues".

During the House inquiry, one of Mr Bolton's deputies, Dr Fiona Hill, testified that Mr Bolton was alarmed by the idea that military aid or a White House meeting between Mr Trump and Ukrainian president Volodymyr Zelensky would be linked to the announcement of investigations.

According to Dr Hill, Mr Bolton told her that he was not involved in the arrangement, which he described as a "drug deal" being "cooked up" by White House Acting Chief of Staff Mick Mulvaney and the president's personal attorney, former New York City Mayor Rudy Giuliani.

The House voted to approve two articles of impeachment against Mr Trump last month, but House Speaker Nancy Pelosi has not yet transmitted those articles to the Senate because of concerns that Senate Majority Leader Mitch McConnell would not conduct a fair trial.

Senate Minority Leader Chuck Schumer has called for the Senate to call several witness -- including Mr Bolton -- at the president's trial, but so far Mr McConnell has not been willing to guarantee any such thing.

In a statement, Mr Schumer called Mr Bolton's willingness to testify a reflection of continuing "momentum for uncovering the truth" in the Senate's eventual trial of Mr Trump and said that any Republican senators who opposed a subpoena for the ex-Trump aide or his papers "would be participating in a cover-up" in light of statements from Mr Bolton's lawyers acknowledging that he has relevant information to share.

"John Bolton correctly acknowledged that he needs to comply with a Senate subpoena to compel his testimony, if issued. It is now up to four Senate Republicans to support bringing in Mr Bolton, and the other three witnesses, as well as the key documents we have requested to ensure all the evidence is presented at the onset of a Senate trial," Mr Schumer said.