Senate minority leader said he aims to compel the chamber as early as Tuesday to hold vote on witnesses and extra evidence

Senate Democrats were ramping up pressure for witnesses to be called in the impeachment trial of Donald Trump before the historic process gets fully under way on Capitol Hill on Tuesday – even as the president plans to be away at the elite economic and political forum in Davos, Switzerland, during opening arguments.

Trump already has no plans to appear at his own Senate trial and will be represented by a legal team that includes some controversial figures. But the symbolism of his being out of the country at a very see-and-be-seen annual event that draws global leaders while he stands accused back home of abusing the power of his office and obstructing Congress during its investigation into his conduct with Ukraine is telling.

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He is due to give a keynote speech at the forum as the drama unfolds in Washington.

The Senate minority leader, Chuck Schumer, said that he aims to compel the chamber as early as Tuesday to hold a vote on whether witnesses and extra evidence can be brought before the impeachment trial, and he turned up the pressure on moderate Republicans to side with the minority to achieve that.

In a press conference in New York on Sunday evening, Schumer said he stood ready to “force votes for witnesses and documents” at the trial if the Senate majority leader, Mitch McConnell – who has yet to set out the exact details of how he proposes that the trial will proceed – does not call for such a vote.

“We have the right to do it, we are going to do it and we are going to do it at the beginning on Tuesday if leader McConnell doesn’t call for these witnesses in his proposal,” Schumer said.

Trump has been keen for the Senate to dismiss the charges against him without even going through the motions of a trial. McConnell intends to proceed with a trial but there has been no agreement to call any outside witnesses, such as the former national security adviser John Bolton or the acting chief of staff, Mick Mulvaney, who could shed even more light on the president’s dealing with the Ukrainians and the pivotal role of Trump’s lawyer Rudy Giuliani.

The case hinges on a 25 July phone call between Trump and the Ukrainian president, Volodymyr Zelenskiy, in which Trump asked his counterpart to do him a “favor” and investigate both a conspiracy theory concerning election interference and ties between the former vice-president Joe Biden and his son Hunter and the eastern European country.

Q&A How do you impeach the US president? Show Hide Article 1 of the United States constitution gives the House of Representatives the sole power to initiate impeachment and the Senate the sole power to try impeachments of the president. A president can be impeached if they are judged to have committed "treason, bribery, or other high crimes and misdemeanors" – although the US Constitution does not specify what “high crimes and misdemeanors” are.

The formal process starts with the House of Representatives passing articles of impeachment, the equivalent of congressional charges. According to arcane Senate rules, after the House notifies the Senate that impeachment managers have been selected, the secretary of the Senate, Julie Adams, tells the House that the Senate is ready to receive the articles. Then impeachment managers appear before the Senate to “exhibit” the articles, and the Senate confirms it will consider the case. The presiding officer of the Senate notifies the supreme court chief justice, John Roberts, of the impending trial. Roberts arrives in the Senate to administer an oath to members. The presiding officer will then administer this oath to senators: “I solemnly swear that in all things appertaining to the trial of the impeachment of Donald Trump, now pending, I will do impartial justice according to the constitution and laws, so help me God.” The Senate must vote on a resolution laying out ground rules for the trial including who the key players will be, how long they will get to present their cases and other matters. After the Senate is “organized”, the rules decree, “a writ of summons shall issue to the person impeached, reciting said articles, and notifying him to appear before the Senate upon a day and at a place to be fixed by the Senate”. A president has never appeared at his own impeachment trial. Trump will be represented by the White House counsel, Pat Cipollone, and his personal lawyer Jay Sekulow, among others. After the oath, the trial proper will begin. Senators may not speak during the proceedings but may submit written questions. The question of witnesses and other matters would be decided on the fly by majority vote. A time limit for the proceedings will be established in the initial Senate vote. The senators will then deliberate on the case. In the past this has happened behind closed doors and out of public view. The senators vote separately on the two articles of impeachment – the first charging Trump with abuse of power, the second charging him with obstruction of Congress. A two-thirds majority of present senators – 67 ayes if everyone votes – on either article would be enough to convict Trump and remove him from office. But that would require about 20 Republicans defections and is unlikely. The more likely outcome is a Trump acquittal, at which point the process is concluded. Two presidents have previously been impeached, Bill Clinton in 1998, and Andrew Johnson in 1868, though neither was removed from office as a result. Richard Nixon resigned in 1974 before there was a formal vote to impeach him. Tom McCarthy in New York

Trump’s team is entitled to file a detailed legal brief in the case before midday on Monday. An initial brief on Saturday argued fiercely against the impeachment charges.

“This is a brazen and unlawful attempt to overturn the results of the 2016 election and interfere with the 2020 election, now just months away,” Trump’s lawyers said, also claiming the charges against the president were invalid as they did not concern a crime.

Schumer indicated he was determined to press for witnesses and documents that have been blocked to be brought forward either before or after opening arguments.

“We’re allowed to amend it, and ask for them. I am allowed to amend it – and then if they say well let’s wait and hear the arguments we’ll want a vote after they hear the arguments as well and we will do everything we can to force votes again,” he said.

On Saturday, the House’s selected team of impeachment managers, who prosecute the case against Trump at trial, outlined their case in a 111-page legal brief.

In a joint statement, the seven managers, led by the Democratic intelligence committee chair, Adam Schiff, said their case was “simple, the facts are indisputable, and the evidence is overwhelming: President Trump abused the power of his office to solicit foreign interference in our elections for his own personal political gain, thereby jeopardising our national security, the integrity of our elections, and our democracy.

“And when the president got caught, he tried to cover it up by obstructing the House’s investigation into his misconduct.”

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On Sunday, the Republican senator Lindsey Graham, once a harsh critic and now a staunch Trump ally, said the latest hope from the president is that he wants the trial and his expected acquittal at the hands of the Republican Senate majority to be out of the way by the annual presidential showcase event, the State of the Union address to Congress and the American people.

“His mood is, to go to the State of the Union [on 4 February] with this behind him and talk about what he wants to do for the rest of 2020 and what he wants to do for the next four years,” Graham told Fox News Sunday.