Democrats seeking to remove United States President Donald Trump from office via impeachment wrapped up three days of opening arguments late on Friday, urging Republicans to allow witnesses and new evidence to be part of the Senate trial.

"Give America a fair trial," said Adam Schiff, the House of Representatives lead manager, at the end of his closing arguments before the Senate.

"She is worth it," he added.

Over three often-gruelling days of argument, the Democratic managers from the House of Representatives attempted to persuade members of the US Senate, who will ultimately decide Trump's fate, that the president is an out-of-control autocrat willing to sacrifice his country's national interest to further his own political ambitions.

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Trump was impeached on December 18 for abuse of power related to his dealings with Ukraine and obstruction of Congress for refusing to participate in the House impeachment inquiry. He has denied any wrongdoing and repeatedly called the impeachment a "witch-hunt".

As the trial enters a new phase on Saturday with Trump's defence team taking the podium, here is a look at how the House managers' opening arguments unfolded:

Day 1

During the first hours of opening arguments on Tuesday, Schiff used video clips, along with screengrabs of text messages and emails, to construct a timeline of how Trump withheld nearly $400m in military aid to pressure Ukrainian President Volodymyr Zelenskyy into announcing an investigation into former Vice President Joe Biden and the origins of foreign interference in the 2016 US presidential election.

Lead manager House Intelligence Committee Chairman Adam Schiff speaking during the continuation of opening arguments on the third day of the Senate impeachment trial of US President Donald Trump [US Senate TV/Handout via Reuters]

Schiff urged the senators to allow for testimony from witnesses, among them former National Security Advisor John Bolton and acting White House Chief of Staff Mick Mulvaney, and for new evidence to be introduced during the proceedings - a prospect that the Republican majority resisted the previous day. Schiff said it was incumbent upon lawmakers to learn the whole story because "the truth is going to come out".

"More emails are going to come out," Schiff said. "More witnesses are going to come forward. They're going to have more relevant information to share. And the only question is, do you want to hear it now? Do you want to know the full truth?"

Day 2

On the second day of their opening arguments, House managers honed in on the question of whether an actual crime has to be committed before a president can be impeached and on whether Biden's actions in Ukraine justified the Trump administration's demands for an investigation.

Anticipating what is almost certain to be a central tenet of the Trump defence, the Democrats teed up a number of videotaped comments from constitutional scholars and even Trump supporters - Attorney General William Barr and Trump defence team lawyer Alan Dershowitz among them - stating in the past that a statutory crime was not a prerequisite of impeachment.

"Impeachment is not a punishment for crimes," New York Democrat Jerrold Nadler told the body. "Impeachment exists to address threats to the political system."

In attempting to justify the first article of impeachment, that Trump abused his power, Nadler cited what he called a "trifecta" of high crimes and misdemeanours by Trump - abuse of his office to solicit election interference, betrayal of US national security interests and the intent to corrupt the 2020 presidential election.

Nadler and his colleagues also spent a fair part of the day arguing that there was no evidence the Bidens acted improperly in Ukraine - pre-empting another likely argument from the president's defence team. They argued that Biden was carrying out official US anti-corruption policies when he sought the dismissal of Ukraine's top prosecutor, Viktor Shokin. Trump, they said, only became interested in Ukrainian corruption after Biden announced that he was running for president in 2020.

"There was no basis for the investigation that the president was pursuing and pushing. None. He was doing it only for his own political benefit," US Representative Sylvia Garcia said.

During a break in the proceedings on Thursday, Republicans said the House managers may live to regret dwelling on Biden's role in the affair. Trump's lawyer said that by doing so they made Biden a relevant subject for the rest of the trial.

"They opened the door. They opened the door and it's now relevant," Jay Sekulow, a personal lawyer for the president and a member of his defence team, told reporters. "So we will address the appropriate issues as defence lawyers would."

Trump's personal lawyer Jay Sekulow speaks to the media during a break in the impeachment trial of President Donald Trump [Jacquelyn Martin/AP Photo]

As the second day wore on, the hours appeared to take their toll on the senators in the chamber. There were more and more empty seats as legislators traipsed back and forth to the cloakrooms and noted that some senators were seen reading books or dozing at their desks.

Senator Lindsey Graham, one of Trump's most ardent supporters, described the proceedings as "mind-numbing". The Democrats, he said, were "over-trying their case".

Day 3

During their final day of arguments on Friday, the Democrats made the case for the second article of impeachment - obstruction of Congress. House leaders said the president repeatedly stonewalled congressional committees during their investigation, and that he would continue to do so unless brought to heel by the Senate.

In this artist sketch, Democratic presidential candidate, Senator Bernie Sanders, flanked by Senator Ben Cardin and Senator Tammy Baldwin, listens during the impeachment trial of President Donald Trump [Dana Verkouteren/AP Photo]

"Merely exposing the president's scheme has not stopped him from continuing his destructive pattern of behaviour that has brought us to this sombre moment," Schiff said. "He is who he is. That will not change. And nor will the danger associated with him. Every piece of evidence supports that terrible conclusion. That the president of the United States will abuse his power again."

What's next?

Trump's defence team, led by White House Counsel Pat Cipollone, are expected to begin their efforts to exonerate the president during a shortened session on Saturday and continue on Monday and Tuesday next week. The team will have 24 hours in total to make its case, the same amount of time allotted to the Democrats.

Co-counsel Jay Sekulow described the Saturday hearing as a preview of things to come next week.

"We have three hours to put it out, so we'll take whatever time is appropriate during that three hours to kind of lay out what the case will look like," Sekulow told reporters during a break in Friday's proceedings. "But next week is when you see the full presentation."

For his part, Trump has taken to his favourite social media platform repeatedly throughout the trial, labelling the impeachment a "hoax" and railing against House managers.