Former White House national security advisor John Bolton has said he is willing to testify if subpoenaed in the Senate impeachment trial of US President Donald Trump.
Democrats believe Mr Bolton has direct knowledge supporting charges of abuse of power and obstruction by the president.
But so far Republican Senate chief Mitch McConnell has indicated he does not want to call witnesses in the trial, expected this month.
"Based on careful consideration and study," Mr Bolton said in a statement, "I have concluded that, if the Senate issues a subpoena for my testimony, I am prepared to testify."
Mr Bolton is one of four Trump administration officials that Democrats want to testify in the trial, which is expected this month.
The House of Representatives has accused Mr Trump of using military aid and other incentives as leverage to get Ukraine to investigate his political rivals, notably possible 2020 White House opponent Joe Biden.
The White House claimed executive privilege to prevent the four – Mr Bolton, White House Chief of Staff Mick Mulvaney, Trump assistant Robert Blair, and budget official Michael Duffey - from testifying to the House impeachment investigation.
If forced to testify in the Senate trial, Republicans fear they could provide deeply damaging evidence against Mr Trump, raising the risk that he will be convicted and removed from office.
Mr Bolton's willingness aside, his subpoena and testimony is not a given.
Republicans hold a 53-47 majority in the Senate and most are expected to support Mr Trump in the trial.
The former National Security advisor to US President @realDonaldTrump has said he will testify in the upcoming Senate #impeachment trial if he is subpoenaed. @rtenews https://t.co/hq9l55nddM
— Brian O'Donovan (@BrianOD_News) January 6, 2020
Issuing a subpoena would require majority support from the senators, meaning several Republicans would have to cross over and join Democrats to vote in favour.
Mr Bolton also noted that the constitutional issue of Mr Trump's ability to block his testimony was not resolved in court before the House voted to impeach Mr Trump on 18 December.
Even without a court ruling on the issue, suggested Mr Bolton, he would be ready to come forward.
It was not clear whether the White House would again claim executive privilege to try to prevent his testimony.
