The public testimony by four experts on constitutional law comes one day after the House Intelligence Committee laid out the case for removing Mr Trump.
If impeachment passes in the House, the US Senate will put Mr Trump on trial.
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 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”.
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.
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 they declined.
Mr Trump is scheduled to return to Washington later on Wednesday, after the first Judiciary hearing has concluded.
What has Trump said about them?
“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 is Trump accused of?
Democrats say Mr Trump dangled two bargaining chips to Ukraine – $400m (£309m) of military aid that had already been allocated by Congress, and a White House meeting for Mr Zelensky – to obtain the investigations. They think this political pressure on a vulnerable US ally amounts to an abuse of power.
The first investigation Mr Trump wanted from Ukraine was into former Vice-President Joe Biden, his main Democratic challenger, and his son Hunter. Hunter joined the board of a Ukrainian energy company when Joe Biden was US vice-president.
The second Trump demand was that Ukraine try to corroborate a conspiracy theory that Ukraine, not Russia, interfered in the last US presidential election.
This theory has been widely debunked, and US intelligence agencies are unanimous in saying Moscow was behind the hacking of Democratic Party emails in 2016.
How does impeachment work?
Impeachment is the first part – the charges – of a two-stage political process by which Congress can remove a president from office.
If, following the hearings, the House of Representatives votes to pass articles of impeachment, the Senate is forced to hold a trial.
A Senate vote requires a two-thirds majority to convict and remove the president – unlikely in this case, given that Mr Trump’s party controls the chamber.
Only two US presidents in history – Bill Clinton and Andrew Johnson – have been impeached, but neither was convicted.
President Richard Nixon resigned before he could be impeached.