In a surprise move, former national security adviser John Bolton said on Monday that he would be willing to testify in impeachment proceedings against US President Donald Trump if called to do so.
"If the Senate issues a subpoena for my testimony, I am prepared to testify," Bolton said in a prepared statement.
Bolton, who was fired from his post in September, would become the highest-ranking White House official to appear in impeachment proceedings against Trump.
The announcement is a major boost for Democrats, who have been pushing for Bolton and other senior Trump administration officials to be summoned to testify in the upcoming impeachment trial in the Senate.
Despite his willingness to testify, the Democrats would need some Republican support for calling him to appear.
Issuing a subpoena in the Senate would require a simple majority of lawmakers in the Republican-controlled Senate — meaning several of Trump's fellow Republicans would have to vote in favor of the move.
Mitch McConnell, the Republican leader in the Senate, has also indicated he does not want to call new witnesses in the trial.
The White House has also blocked testimony before by citing executive privilege, while telling others to ignore the subpoenas — which Congress has few options to enforce.
Trump focus on impeachment, amid Iran upheaval
Despite rising tensions with Iran, President Trump has reserved much of his social media ire on Monday for the impeachment proceedings against him.
In several posts on Twitter on Monday, Trump repeatedly called the proceedings against him as a "hoax" and a "witch hunt."
Impeachment proceedings have stalled over the witness issue. House of Representatives Speaker and top Democrat Nancy Pelosi has not yet formally sent the articles of impeachment to the Senate — meaning the trial schedule is unclear.
The Democrat-controlled House of Representatives voted to impeach Trump on December 18 on charges of abuse of power and obstruction.
Why Bolton?
Based on testimony from other witnesses in impeachment proceedings, Democrats believe Bolton has direct knowledge that supports the charges against Trump.
Trump is accused of withholding military aid and pressuring Ukraine to investigate his political rival Joe Biden.
Fiona Hill, a former White House official, previously testified that Bolton called the scheme a "drug deal" and told her to notify lawyers for the National Security Council.
Hill also testified that Bolton called Trump's personal lawyer Rudy Giuliani, who played a central figure in the pressure campaign, a "hand grenade that is going to blow everybody up."