Republicans angrily retorted there were no grounds for such drastic action.

The panel responsible for drafting articles of impeachment convened as Trump's team was fanning out across Capitol Hill.

Vice President Mike Pence met behind closed doors with House Republicans, and Senate Republicans were due to huddle with the White House counsel as Republican politicians stand with the president and Democrats charge headlong into what has become a one-party drive to impeach him.

Chairman Jerrold Nadler, a Democrat, opened the hearing, saying: "The facts before us are undisputed."

Nadler said Trump's phone call with Ukraine's president last July was not the first time Trump had sought a foreign power to influence American elections, after alleged Russian interference in 2016, and if left unchecked he could do so again in next year's campaign.

"We cannot wait for the election to address the present crisis," Nadler said.

"The president has shown us his pattern of conduct. If we do not act to hold him in check now, President Trump will almost certainly try again to solicit interference in the election for his personal political gain."

Republicans protested that the proceedings were unfair to the president, with the dredging up of unfounded allegations as part of an effort to undo the 2016 election and remove Mr Trump from office.

"You just don't like the guy," said Doug Collins, the top Republican on the panel.

He called the proceedings a "disgrace" and a "sham".

Several Republicans immediately objected to the process, interjecting procedural questions, and they planned to spend much of the session interrupting, delaying and questioning the rules.

"This is not impeachment, this is a simple railroad job," Collins said.

House Speaker Nancy Pelosi said Democrats "haven't made a decision" yet on whether there will be a vote on impeachment.

She was also meeting privately with the Democratic caucus.

But a vote by Christmas appears increasingly likely after the release of a 300-page report by Democrats on the House intelligence committee that found "serious misconduct" by the president.

"The evidence that we have found is really quite overwhelming that the president used the power of his office to secure political favours and abuse the trust American people put in him and jeopardise our security," intelligence committee chairman Adam Schiff, a Democrat, said.

"Americans need to understand that this president is putting his personal political interests above theirs. And that it's endangering the country."

The judiciary committee heard on Wednesday from legal experts to determine whether Trump's actions stemming from the July 25 phone call with Ukraine's president rose to the constitutional level of "high crimes and misdemeanours" warranting impeachment.

The three legal experts called by Democrats backed impeachment.

Noah Feldman, a Harvard Law School professor, said he considered it clear that the president's conduct met the definition of "high crimes and misdemeanours".

Pamela Karlan, a Stanford Law School professor and former Obama administration Justice Department official, said the president's action constituted an especially serious abuse of power "because it undermines democracy itself".

Republican witness Jonathan Turley, a law professor at George Washington University, said that the Democrats were bringing a "slipshod impeachment" case against the president but he did not excuse Trump's behaviour.

"It is not wrong because President Trump is right," according to Turley. "A case for impeachment could be made but it cannot be made on this record," he said.

The report laid out evidence that the Democrats say show Trump's efforts to seek foreign intervention in the US election.

New telephone call records released with the report deepen Trump lawyer Rudy Giuliani's known involvement in what House investigators called the "scheme" to use the president's office for personal political gain by enlisting a foreign power, Ukraine, to investigate Democrats including Joe Biden, and intervene in the American election process.

Trump told reporters in London, where he was attending a NATO meeting, he really does not know why Giuliani was calling the White House's Office of Management and Budget, which was withholding $US400 million in military aid to the ally confronting an aggressive Russia at its border.

"You have to ask him," Trump said.

"Sounds like something that's not so complicated. ... No big deal."

Australian Associated Press