US President Donald Trump leaves the White House in Washington D.C. December 18, 2019. The US House of Representatives passed the impeachment against Donald Trump on Wednesday. Photo: Xinhua

US President Donald Trump's former national security adviser John Bolton said Monday he is willing to testify in the expected Senate impeachment trial of the president, a surprise development that could potentially strengthen the case that Trump should be removed from office.

As a top White House aide who witnessed many of the events that ­prompted the House of Representatives to impeach Trump in December, Bolton could provide new evidence about Trump's efforts to pressure Ukraine to investigate a political rival.

Other witnesses during the House impeachment investigation testified that Bolton strongly objected to an effort by Trump's personal lawyer, Rudy Giuliani, to pressure Kiev outside of regular diplomatic channels, with one saying he referred to the arrangement as a "drug deal."

Congressional investigators believe Bolton objected to Trump's decision to delay $390 million in military aid to Ukraine and could elaborate on that, a Senate aide told Reuters.

Bolton's lawyer said in November that he could shed new light on White House discussions, but Bolton refused to participate in the House impeachment inquiry while the Trump administration and Congress battled in court for access to witnesses and documentary evidence.

Bolton said he was now willing to cooperate after a judge dismissed the case last week.

"If the Senate issues a subpoena for my testimony, I am prepared to testify," Bolton said in a statement. He declined further comment.

Democrats seized on Bolton's announcement, saying it bolstered their argument that he and three current administration officials should testify when the Senate begins its impeachment trial.

"If any Senate Republican opposes issuing subpoenas to the four witnesses and documents we have requested they would make absolutely clear they are participating in a coverup," Senate Democratic leader Chuck Schumer said.

Trump's fellow Republicans have resisted that idea, instead seeking a quick trial based on the evidence collected in the House that could lead to the president's expected acquittal before the 2020 presidential election campaign heats up.

In remarks on the Senate floor, Republican Senate majority leader Mitch McConnell repeated his proposal that the chamber postpone a decision on whether to call witnesses until after the trial begins. That could allow Republicans, who control the chamber by a margin of 53-47, to wrap up the trial without hearing from Bolton or other witnesses.