WASHINGTON (AP) — House investigators are asking former national security adviser John Bolton to testify in their impeachment inquiry, deepening their reach into the White House as the probe accelerates toward a potential vote to remove the president.

Democratic lawmakers want to hear next week from Bolton, the hawkish former adviser who openly sparred over the administration’s approach to Ukraine — in particular, President Donald Trump’s reliance on his personal attorney Rudy Giuliani for a back-channel operation. Bolton once derided Giuliani’s work as a “drug deal” and said he wanted no part of it, according to previous testimony.

Bolton’s attorney, Charles Cooper, said Wednesday evening that his client would not appear without a subpoena.

The Democrats are also calling John Eisenberg, the lawyer for the NSC who fielded an Army officer’s concerns over Trump’s phone call with the Ukraine president, and Michel Ellis, another security council official, according to a person familiar with the invitation and granted anonymity to discuss it.

The rush of possible new witnesses comes as the House prepares to take its first official vote Thursday on the process ahead. That includes public hearings in a matter of weeks and the possibility of drafting articles of impeachment against the president.

The White House has urged officials not to testify in the impeachment proceedings, and it’s not guaranteed that those called will appear for depositions, even if they receive subpoenas as previous witnesses have.

Bolton’s former deputy, Charles Kupperman, has filed a lawsuit in federal court asking a judge to resolve the question of whether he can be forced to testify since he was a close and frequent adviser to the president. Any ruling in that case could presumably have an impact on whether Bolton will testify.

Trump and his Republican allies on Capitol Hill say the entire impeachment inquiry is illegitimate and are unpersuaded by the House resolution formally setting out next steps.

Senate Majority Leader Mitch McConnell said the format for the impeachment probe denies Trump the “most basic rights of due process.”

Now in its second month, the investigation is focused on Trump’s July phone call with Ukraine when he asked President Volodymyr Zelenskiy to investigate Democrats and a potential 2020 political rival, Joe Biden, as the White House was withholding military aid Ukraine relies on for its defenses. Democrats contend Trump was proposing a quid-pro-quo arrangement.

On Thursday, the investigators are to hear from Tim Morrison, a former top GOP aide on Capitol Hill, who served at Trump’s National Security Council and was among those likely monitoring the president’s call with Ukraine.

Late Wednesday, it was disclosed that Morrison was resigning his White House position. He has been a central figure in other testimony about Trump’s dealing with Ukraine.