The impeachment investigation into Donald Trump has entered a new phase as the House Judiciary Committee began hearings into his conduct. Four experts on constitutional law, three picked by Democrats and one by the Republicans, are giving their take on whether he should be impeached. The lone Republican-backed professor said Mr Trump's actions were wrong, but not impeachable. The other three have said there is no doubt that his actions require removal. The impeachment process began in September after an anonymous whistleblower complained to Congress about a July phone call by Mr Trump to the president of Ukraine, in which Mr Trump appeared to tie US military assistance to Ukraine launching investigations which would help him politically. The White House has denied allegations made by Democrats - that Mr Trump put his own personal political interests "above the national interests of the United States" by soliciting foreign interference in the 2020 US elections, as the 300-page intelligence committee report argues. After the report's release on Tuesday, White House spokeswoman Stephanie Grisham said that it "utterly failed to produce any evidence of wrongdoing". Speaking from the UK, where he is attending a Nato summit, Mr Trump questioned the patriotism of Democrats. "You almost question whether or not they love our country and that's a very serious thing: Do they love our country?" he said on Wednesday. Among formal impeachment charges expected to be considered by the judiciary committee are abuse of power, obstruction of justice and contempt of Congress. Democrats are keen to hold a vote on impeachment in the House of Representatives before the end of the year, with the prospect of a trial in the Senate perhaps as early as January 2020. What is the judiciary committee's role? The judiciary committee has the power to formally draft articles of impeachment and submit them for a full vote in the House of Representatives. The committee is hearing on Wednesday from four law professors - three picked by Democrats and one by Republicans. Chosen by the Democrats are Stanford University's Pamela Karlan, Harvard University's Professor Noah Feldman and from the University of North Carolina, Michael Gerhardt. George Washington University's Jonathan Turley was picked by Republicans. The lawyers will interpret the impeachment clause of the constitution, which allows for presidents to be removed from office due to "high crimes and misdemeanours". The White House was invited to participate in the hearing, but on Sunday declared that they would not send any administration officials to attend. Mr Trump was scheduled to return from London to Washington later on Wednesday, after the first judiciary hearing has concluded. But on Wednesday, Mr Trump announced that he would depart early, skipping a final news conference "because we did so many over the past two days". His hasty departure came soon after a video emerged of other world leaders at the Nato conference appearing to mock him. What has Trump said about the experts? "They get three constitutional lawyers... and we get one," Mr Trump said on Tuesday during a bilateral meeting with Canadian Prime Minister Justin Trudeau in London. "That's not sounding too good, and that's the way it is. "It's all nonsense, just wasting their time, and we get one. Ok. Nobody needs to know anything about constitutional law," he said. What have the experts said? Prof Feldman testified that the "evidence clearly constitutes" an impeachable offence because Mr Trump's interactions with Ukraine show him "corruptly using the powers of the presidency for personal political gain". As she began her testimony, Prof Karlan scolded the committee's top Republican, saying she was "insulted" by his comment implying that she had not reviewed the intelligence committee's public testimony of 12 witnesses. "I would like to say to you, sir, that I read transcripts of every one" of the witnesses, she said. "But everything I read on those occasions tells me that when President Trump invited, indeed demanded, foreign involvement in our upcoming election, he struck at the very heart what makes this a republic to which we pledge allegiance." Mr Trump has attacked the "safeguards against establishing a monarchy in this country", Prof Gerhardt stated. "The president's serious misconduct, including bribery, soliciting a personal favour from a foreign leader in exchange for his exercise of power, and obstructing justice and Congress are worse than the misconduct of any prior president, including what previous presidents who faced impeachment have done or been accused of doing," he said in his opening remarks. Prof Turley, who was chosen as a witness by Republicans, said he disagreed with Mr Trump's conduct but "this is not how an American president should be impeached". He also warned that Democrats are setting a dangerous precedent. "I get it. You are mad. The President is mad. My Democratic friends are mad. My Republican friends are mad...." he said. "We are all mad and where has it taken us? Will a slipshod impeachment make us less mad or will it only give an invitation for the madness to follow in every future administration?"