Protesters rally calling for the removal of President Donald Trump inside of the Hart Senate Office Building during a demonstration on Capitol Hill in Washington US on January 16, 2020 Reuters

The fast-moving impeachment of Trump, stemming from his dealings with Ukraine, has moved to the Senate for trial after the House voted last month to adopt two articles of impeachment: abuse of power and obstruction of Congress

For only the third time in history, a US president is facing an impeachment trial, with hearings start on Tuesday at 18:00 GMT. Such a trial could, in theory, lead to President Donald Trump being removed from office.

The fast-moving impeachment of Trump, stemming from his dealings with Ukraine, has moved to the Senate for trial after the House voted last month to adopt two articles of impeachment: abuse of power and obstruction of Congress.

The vote followed weeks of testimony related to the president's efforts to press Ukraine for investigations into Democratic rivals and hours of fiery debate over the process. Trump is only the third president in US history to be impeached. 

Backers and critics clash

Trump's defenders and supporters skirmished over the airwaves on Sunday a day after the US president's legal team dismissed his impeachment trial as unconstitutional and dangerous.

Beginning Tuesday, the chamber will meet six hours a day for six days a week in only the third impeachment trial of a US president, with lofty constitutional issues brushing up against raw partisan politics.

It will be a "gruelling exercise," Republican Senator John Cornyn said on CBS.

Celebrity attorney Alan Dershowitz, a recent addition to Trump's legal team, argued Sunday that even if every charge sent by the House to the Senate for the president's trial were accepted as true, it would not rise to the level of impeachable behaviour.

"The (House) vote was to impeach on abuse of power, which is not within the constitutional criteria for impeachment, and obstruction of Congress," Dershowitz, a Harvard law professor emeritus, said on ABC.

Adam Schiff, the California lawmaker chosen by House Democrats as lead manager of the impeachment trial, dismissed the notion that abuse of power was not impeachable.

'Arrant nonsense'

"That's an argument you have to make if the facts are so dead set against you," he said on ABC. 

Another House impeachment manager, Jerrold Nadler, called Dershowitz's argument "arrant nonsense."

For Republicans to argue that such behaviour is not impeachable, Schiff said, "would have appalled the founders, who were worried about exactly that kind of solicitation of foreign interference in an election for personal benefit."

The two sides have been publicly fencing over whether the trial will be conducted quickly, perhaps in as little as two weeks - Trump's clear preference - or whether

witnesses can be called and new evidence introduced - as Democrats insist is needed for a full and fair trial.

The president has said he would like the Senate to almost immediately dismiss the charges, but Republican Senator Lindsey Graham, a Trump confidant, told Fox News: "That is not going to happen. We don't have the votes for that."

Possible delays

Dershowitz said Sunday, however, that calling witnesses would introduce major delays. 

"The trial will take a much longer time, because the Democrats will call (former national security advisor) John Bolton, and the president will invoke executive privilege and it will have to go to the courts," he said on CNN.

Republicans say the House erred by not taking the time during its hearings to secure the testimony of key witnesses like Bolton before sending articles of impeachment to the Senate. 

Schiff said that would have meant "endless months or even years of litigation."

He added, "What the president is threatening to do is cheat in the next election. You cannot wait months and years to be able to remove that threat from office."

Trump himself expressed confidence Sunday that the Republican majority in the Senate would remain solid and acquit him.

"I have never seen the Republican Party as Strong and as Unified as it is right now. Thank you!" he said on Twitter.