The US House of Representatives will hold its first formal vote Thursday on the impeachment inquiry into President Donald Trump, a senior aide said, as Democrats forge ahead with a process that includes public hearings.

The move marks a shift in strategy by Democrats who had insisted for weeks that they did not need a floor vote to proceed with investigation into Trump's dealings with Ukraine, a posture that angered Republicans who relentlessly attacked the process as unfair.

The chamber will go on record for the first time to "lay out the next steps for the inquiry," a senior Democratic aide told AFP Monday after House Speaker Nancy Pelosi informed fellow Democrats of the plan.

The effort appears aimed in part at pushing back against Republican criticism that the investigation was not providing Trump with due process.

"This week, we will bring a resolution to the floor that affirms the ongoing, existing investigation that is currently being conducted by our committees as part of this impeachment inquiry," Pelosi said in a letter to her caucus.

The measure is likely to pass in the Democratic-controlled House given that 228 Democrats, out of a total 435 House members, are on record supporting impeachment or an impeachment inquiry.

The measure "establishes the procedure for hearings that are open to the American people, authorizes the disclosure of deposition transcripts (and) outlines procedures to transfer evidence to the Judiciary Committee as it considers potential articles of impeachment," she added.

To date, all witness testimony in the month-long inquiry has been taken behind closed doors, leading Republicans to slam the process -- without merit -- as secretive and illegitimate.

'Eliminate any doubt'

The resolution, Pelosi said, also "sets forth due process rights for the President and his Counsel" -- steps that Republicans had repeatedly stressed were being ignored by the House's majority party.

"We are taking this step to eliminate any doubt as to whether the Trump administration may withhold documents, prevent witness testimony, disregard duly authorized subpoenas, or continue obstructing the House of Representatives," she added.

The White House reacted swiftly.

Pelosi "is finally admitting what the rest of America already knew -- that Democrats were conducting an unauthorized impeachment proceeding, refusing to give the president due process, and their secret, shady, closed door depositions are completely and irreversibly illegitimate," Press Secretary Stephanie Grisham said.

Trump himself retweeted his top House lieutenant, Minority Leader Kevin McCarthy, who said "today's backtracking is an admission that this process has been botched from the start."

In fact, depositions are often taken in private, to avoid witnesses aligning their stories, and this was the practice during Watergate and, more recently, in the Republicans' Benghazi investigations.

The House rules allowing for closed-door hearings limited to members of the investigating committees in a secure room were passed by Congress in 2015, when it was controlled by the Republicans.

Pelosi, Trump's chief adversary in Congress, made her move as the impeachment showdown took an urgent new tone in Washington with a key witness defying a US House subpoena to testify.

The White House has sought to block the appearance by ex-deputy national security advisor Charles Kupperman, who last week took the rare step of filing a lawsuit urging a court to rule on whether he should obey the commands of the executive branch or the legal imperatives of Congress.

Democrats are keen to hear from Kupperman, as he reportedly was on the July 25 telephone call between Trump and his Ukrainian counterpart in which the US leader pushed Kiev to investigate his political opponents.

That request by Trump, and accusations he conditioned nearly $400 million in military aid to Ukraine on the political favor, form the basis of the impeachment inquiry that began five weeks ago and now threatens his presidency.

House Rules Committee chairman Jim McGovern said he would introduce the impeachment inquiry "transparency" measure Tuesday, and that the panel would debate it and hold a preliminary vote 3.00 pm (1900 GMT) Wednesday.

Should it clear the committee, it heads to the floor for a full vote the following day.

Copyright AFP. All rights reserved.

Trump formal impeachment
The impeachment inquiry against US President Donald Trump is accelerating, with Democrats announcing a full House vote on a resolution laying out the next steps in the process. Photo: AFP / Brendan Smialowski