Three constitutional experts on Wednesday bolstered Democrats' efforts to impeach Donald Trump by saying the president's actions seeking foreign interference in U.S. elections are grounds for removal, as the inquiry kicked into a new phase in Congress.

But reflecting the Washington political divide, a fourth expert strongly dissented, saying there was "woefully inadequate" evidence that Trump committed high crimes and misdemeanors necessary for impeachment.

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Democrats have made a forceful case that Trump should be removed from office for abusing his powers by pressuring Ukraine for dirt on an election rival, arguing that the Founding Fathers had a remedy - impeachment - for such wrongdoing.

In a theatrical, contentious hearing of the House Judiciary Committee - now tasked with weighing impeachment charges against the president - lawmakers listened to damning testimony from constitutional law professors who said Trump's conduct rises to that level.

"We three are unanimous," jurisprudence professor Michael Gerhart of the University of North Carolina stated, referring to fellow witnesses Pamela Karlan, a Stanford Law School professor, and professor Noah Feldman of Harvard Law School.

Americans tuning in to the live broadcast witnessed heated clashes between Democrats and Republican Trump loyalists on the panel, who repeatedly forced procedural votes to help stall the process.

The new phase of impeachment began a day after a congressional report on the high-stakes inquiry detailed "overwhelming" evidence of abuse of power and obstruction by the president.

Trump "directly and explicitly invited foreign interference in our elections," said House Judiciary Committee Chair Jerry Nadler, referring to Trump's effort to get dirt on his Democratic rivals from Ukraine.

"He used the powers of his office to try to make it happen. He sent his agents to make clear that this is what he wanted and demanded," Nadler said.

"He was willing to compromise our security and his office for personal, political gain."

Doug Collins, the senior Republican on the panel, countered with the suggestion the hearing was a partisan political effort to undermine a democratically elected president.

"We're having a fact-less impeachment," he said, rejecting the final report on the investigation of Trump released on Tuesday.

"We have just a deep-seated hatred of a man who came to the White House and did what he said he was going to do," Collins said, referring to Trump.

"This is not an impeachment. This is just a simple railroad job. And today's is a waste of time."

On Tuesday, congressional Democrats made a forceful case that Trump should be removed from office for abusing his powers by pressuring Ukraine for dirt on a Democratic election rival.

The final report on the House Intelligence Committee's probe, led by lawmaker Adam Schiff, concluded that "the president placed his personal political interests above the national interests of the United States, sought to undermine the integrity of the U.S. presidential election process and endangered U.S. national security."

The report is expected to form the basis for the Judiciary Committee to draw up formal charges – articles of impeachment – that could include bribery, abuse of power, obstruction and contempt of Congress.

Kicking off the next phase of impeachment, four constitutional scholars were called to testify on Wednesday before the House Judiciary Committee in the first of a series of hearings to establish the gravity of Trump's alleged crimes.

In London for a NATO summit, Trump slammed the Democratic report presenting the case for his impeachment as a "joke" and lambasted his opponents for proceeding with hearings during his trip.

"What they are doing is a very bad thing for our country," Trump said when asked about the report. "It's a joke."

Democrats reportedly aim to have articles of impeachment presented for a vote to the entire House of Representatives before Christmas on December 25.

If they pass as expected, Trump would then stand trial for removal in the Republican-controlled Senate, where he is expected to be exonerated.

(With input from AFP)