Ex-US national security adviser John Bolton said on Monday that he is willing to testify in the expected Senate impeachment trial of the president, a surprise development that could complicate a weeks-long dispute over how the trial would play out.

Bolton further said in a statement, “If the Senate issues a subpoena for my testimony, I am prepared to testify”.

Democrats have been pressing for Bolton and three senior administration officials to testify as part of the Senate trial, while Trump’s fellow Republicans are seeking a quick trial that could lead to the president’s expected acquittal before the 2020 presidential election campaign heats up.

“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 cover-up,” Senate Democratic Leader Chuck Schumer said.

Senate Republican Leader Mitch McConnell did not immediately respond to a request for comment.

Last month, Bolton was so alarmed by a White House–linked effort to pressure Ukraine to investigate Democrats, he told aide Fiona Hill to alert the National Security Council’s chief lawyer.

Bolton also warned that President Donald Trump’s personal attorney Rudy Giuliani, who has emerged as the point man in the president’s alleged drive to pressure Ukraine to dig up dirt on Democratic presidential hopeful Joe Biden, is “a hand grenade who’s going to blow everyone up,” his aide Fiona Hill told lawmakers, New York Times reported.

Ukraine’s former president had said that he discussed investments with President Donald Trump’s attorney, Rudy Giuliani, in 2017, but that he never discussed Ukrainian companies with any US official.

In November, President Trump formally moved into its public phase, he lashed out at “crazy” and “corrupt” opponents probing potential abuse of presidential power.

The increasing prospect of becoming only the third US president formally impeached, after Andrew Johnson and Bill Clinton, has riled the brash New York billionaire, and he fired off multiple attacks against rivals leading the effort.

President Trump described the impeachment probe against him as “witch hunt”, saying he was “too busy” to watch it.

In October, President Trump opposed impeachment enquiry, saying that there should be no public hearings during the House of Representatives’ impeachment enquiry against him, and directed White House Chief of Staff Mick Mulvaney not to appear before the committees investigating Trump’s phone call to Ukraine.

After almost a month of calling for greater transparency in the enquiry, the White House changed its strategy this week by prohibiting several of its officials from even testifying behind closed doors before the lower house committees.

Late September, the impeachment inquiry, which House Speaker Nancy Pelosi initiated over a complaint by an anonymous whistleblower, is looking into White House’s alleged efforts to withhold military aid to have Ukraine investigate a Trump’s political rival, Joe Biden.