“I have concluded that, if the Senate issues a subpoena for my testimony, I am prepared to testify,” Bolton said in a statement.

The House of Representatives has accused Trump of using military aid and other incentives as leverage to get Ukraine to investigate Democrat Joe Biden, his potential November presidential opponent.

During the impeachment hearings in the House, National Security Council aides to Bolton said he had told them to talk to White House lawyers following Trump’s efforts to get Ukraine to investigate Biden.

Bolton is one of four Trump administration officials that Democrats want to provide testimony in the Senate trial.

But Bolton, a veteran Washington insider who was fired by Trump in September, has not publicly disclosed whether his testimony would be damaging or helpful to the president.

The White House claimed executive privilege to prevent the four – Bolton, White House Chief of Staff Mick Mulvaney, Trump assistant Robert Blair, and budget official Michael Duffey – from testifying before the House.

If forced to testify in the Senate trial, Republicans fear they could provide deeply damaging evidence against Trump, raising the risk that he will be convicted and removed from office.

Bolton’s willingness aside, his subpoena and testimony were not a given.

Republicans hold a 53-47 majority in the Senate and most are expected to support Trump in the trial.

Issuing a subpoena would require majority support from the senators, meaning several Republicans would have to cross over and join Democrats.

Republican Senator Mitt Romney said he would favour an appearance by Bolton.

“I would like to be able to hear from John Bolton,” Romney said.

“He has first-hand information and that is something that I’d like to hear.”

In his statement, Bolton noted that the constitutional issue of Trump’s ability to block his testimony was not resolved in court before the House voted to impeach Trump on December 18.

Even without a court ruling on the issue, Bolton suggested he would be ready to come forward.

It was not clear whether the White House would again claim executive privilege to try and prevent his testimony.

Citing the impeachment trial of President Bill Clinton in 1999, Mitch McConnell, the Senate majority leader, said that a decision to call witnesses should be made only after the trial has begun.

“And that’s the way it should be done this time,” he said.

He denounced the House impeachment inquiry as “unfair” and accused Democrats of political brinkmanship at a time of rising tensions with Iran.

“Even with tensions rising in the Middle East, House Democrats are treating impeachment like a political toy,” he said. “At this dangerous time, House Democrats continue to play political games with their partisan impeachment of the commander-in-chief.”

Pelosi countered that Trump and McConnell “must allow key witnesses to testify” so Americans can “see the facts for themselves.”

“The Senate cannot be complicit in the president’s cover-up,” she said.

Democratic lawmaker Adam Schiff, who led the House impeachment inquiry, also called for Bolton to testify.

“Bolton is an important witness to misconduct involving Ukraine that he called a ‘drug deal,’” Schiff said. “Bolton refused to testify in the House, following Trump’s orders.

“Now he is willing to come forward,” he said. “The Senate must allow testimony from him, Mulvaney and others. The cover-up must end.”

Tribune News Service and Agence France-Presse