House Democrats unveiled plans Tuesday to open up the impeachment inquiry into President Trump.

WASHINGTON (AFP) - House Democrats unveiled plans Tuesday to open up the impeachment inquiry into President Donald Trump with public hearings, after criticism from Republicans that the process has been overly secretive.

The White House responded that it still considers the proceedings a "scam," signaling no let-up in Trump s strategy of trying to prevent the investigation from going ahead.

Democrats deny that they have been secretive, arguing that five weeks of closed hearings -- in which Republican lawmakers participated -- were necessary for evidence-gathering, ahead of the public portion of the inquiry into the accusation that Trump pressured Ukraine to investigate a political rival.

On Tuesday, Democrats proposed legislation for the next stages of the process, giving Republicans the right to call their own witnesses and subpoena records.

The House Intelligence Committee will then govern the process of firming up evidence from testimony and documents, with both sides equally able to question witnesses in a public setting.

The third stage will see the evidence against Trump forwarded to the Judiciary Committee to draw up articles of impeachment, which would be voted on by the entire House. The Senate would then vote on whether to convict and remove Trump from office.

The rules give Trump and his lawyers their first opportunity to take a direct role and argue their case before the Judiciary Committee, calling for more testimony or evidence and cross-examining witnesses, the House Rules Committee said.

However, it added, if the White House rejects subpoenas for testimony and documents from the probe -- as it has done so far -- it could be denied some of the promised role in the hearings.

That clash does look set to continue.

The "Democrats  impeachment has been an illegitimate sham from the start as it lacked any proper authorization by a House vote," Trump spokeswoman Stephanie Grisham said in a statement after the Democrats  plans were announced.

The revamped procedures do "nothing to change the fundamental fact that House Democrats refuse to provide basic due process rights," she said.

The new rules are slated to be voted on by the Democratic-majority House on Thursday.

"The House impeachment inquiry has collected extensive evidence and testimony, and soon the American people will hear from witnesses in an open setting," senior House Democrats said in a statement.

"The evidence we have already collected paints the picture of a president who abused his power by using multiple levers of government to press a foreign country to interfere in the 2020 election," they added.

The impeachment plan was released as a senior White House national security official presented damning testimony against Trump behind closed doors in Congress Tuesday.

National Security Council Ukraine expert Lieutenant Colonel Alexander Vindman said in prepared testimony that he had personally witnessed a senior US diplomat pressure Ukraine to open investigations designed to help Trump politically.

And he was listening when, in a July 25 phone call, Trump asked Ukrainian President Volodymyr Zelensky to do him a "favor" on the investigations.