Speaker of the House Nancy Pelosi presides over the US House of Representatives vote on a resolution that sets up the next steps in the impeachment inquiry of US President Donald Trump on Capitol Hill in Washington, DC. (REUTERS/Tom Brenner)

"Today the House takes the next step forward as we establish the procedures for open hearings... so that the public can see the facts for themselves," said House Speaker Nancy Pelosi, Washington's top Democrat.

"What is at stake in all of this is nothing less than our democracy."

The House of Representatives voted along party lines, 232 to 196, to pass a resolution that lays out guidelines for the next stages of the impeachment process.

All Republicans opposed the measure, despite agitating for weeks for such a vote that could end the secretive depositions and bring the process into the open.

Democrats are seeking to learn whether Trump abused his presidential power by pressuring a foreign government to investigate a domestic political rival.

Trump has repeatedly branded the inquiry as illegitimate and politically motivated, and he was true to form in his instant reaction to the vote.

"The Greatest Witch Hunt In American History!" he boomed on Twitter.

The White House accused opposition Democrats of being "fundamentally un-American" in their "unhinged obsession with this illegitimate impeachment."

"Democrats are choosing every day to waste time on a sham impeachment - a blatantly partisan attempt to destroy the President," Press Secretary Stephanie Grisham said in the statement after the vote.

Speaker of the House Nancy Pelosi (D-CA) speaks during a media briefing ahead of a House vote authorising an impeachment inquiry into US President Donald Trump. (REUTERS/Joshua Roberts)

Trump meanwhile called on Republicans to rally behind him even as he faced the embarrassing likelihood of becoming the third president in history to be impeached and placed on trial for removal in the Senate, over an alleged scheme to extort Ukraine's help to get him reelected in 2020.

The embattled president retweeted a rallying cry from Fox News host Laura Ingraham that called on Republicans to "stand together and defend the leader of their party against these smears."

'FALSE DEFENCES'

Trump is accused of withholding military aid to compel Ukraine to mount a corruption probe against his Democratic election rival Joe Biden - effectively using US foreign policy in an illegal shakedown for his personal political benefit.

Trump and loyal Republicans dismiss the case as a "sham," but congressional investigators have heard a steady flow of corroborating evidence from government officials testifying behind closed doors on Capitol Hill.

The landmark resolution did not have unanimous Democratic approval. Two Democrats joined all Republicans in opposing the measure, although former Republican Justin Amash, now an independent, supported it - and offered a searing warning to his former party.

"Excusing his misbehavior will forever tarnish your name," he tweeted to Republicans. "History will not look kindly on disingenuous, frivolous, and false defences of this man."

The inquiry now moves into the public eye - giving Americans the chance to hear on live television the evidence against Trump.

The House Intelligence Committee, which has led the inquiry so far, will host open hearings, presenting witnesses and documentary evidence and allowing Republicans to challenge the case against Trump.

The minority can also suggest subpoenas but majority Democrats have final say, a rule that has angered Republicans.

Democrats argue that the vote neutralizes a key Republican talking point that the inquiry has no validity because the full House did not sign on.

Neither the House rules nor the US Constitution require such a vote to start an impeachment inquiry.

If the case against Trump is deemed strong enough, the House Judiciary Committee will draw up formal charges against the president - articles of impeachment - to be voted on by the full House.

It remains unclear how quickly the process can advance. Some say the Democratic-led House could impeach Trump by year end.

He would then be tried in the Senate, controlled by Republicans.

Several of the Democratic 2020 presidential hopefuls praised the move, including Biden.

"Congress must do its duty to ensure that Donald Trump's assault on the Constitution does not seep beyond his presidency," he said.

WITNESSES BACK ALLEGATIONS

Nearly a dozen witnesses so far have confirmed in House depositions the accusations that, in a concerted effort with top aides and his personal lawyer, Trump pressured Ukraine to help his reelection effort in 2020 by producing dirt on Biden, the former vice president.

The allegations focus on a July 25 phone call in which Trump pressed Ukraine President Volodymyr Zelensky to open investigations into Biden and Biden's son, who worked with a Ukraine energy firm.

In potentially damning testimony Thursday, the White House National Security Council's top Russia expert, Tim Morrison, confirmed that a diplomat close to Trump told Morrison that military aid would be withheld until Kiev committed to investigating Biden.

The revelation corroborates testimony provided last week by the top US envoy to Ukraine, Bill Taylor, who expressed concern that Morrison informed him of the quid pro quo with Ukraine.

"I can confirm that the substance of his statement, as it relates to conversations he and I had, is accurate," Morrison, who resigned Wednesday, testified in his opening statement obtained by US media.

Investigators also have summoned Trump's former national security advisor John Bolton to testify.

VNF/AFP