President Donald Trump's historic impeachment trial begins in earnest today in the Senate, with Democrats calling for his removal from office and Republicans determined to acquit him -- and quickly, if possible.

Four months after the Ukraine scandal exploded and went on to overshadow the end of Trump's term, and 10 months before Americans go to the polls to decide whether to re-elect him, the 100 members of the Senate will gather at 1:00pm, Washington time, with Supreme Court Chief Justice John Roberts presiding over the trial.

The job of these lawmakers, sworn in last week as jurors, is to decide if Trump abused his office and obstructed Congress as charged in the two articles of impeachment approved last month by the House of Representatives.

They state that Trump tried to pressure Ukraine into interfering in the 2020 US election to help him win, and then tried to thwart a congressional probe of his behaviour.

It will be only the third time a president has endured an impeachment trial, after Andrew Johnson in 1868 and Bill Clinton in 1999.

Part of the scandal centres on a July 25 telephone call in which Trump pressured Ukrainian President Volodymyr Zelensky to announce an investigation of former Vice President Joe Biden, Trump's potential opponent in the November vote.

Democrats, who control the House of Representatives and led the investigation, accuse Trump of manipulating Ukraine by withholding nearly USD 400 million in military aid for its war against Russian-backed separatists and a White House meeting for Zelensky until the latter announced a Biden probe.

"The president did nothing wrong," Trump's lawyers responded in a 110-page brief submitted to the Senate on Monday.

This echoes the repeated assertions of the president that the saga is a political witch hunt and a hoax, and that his phone call with the Ukrainian leader was "perfect."

In the US President Donald Trump's brief, his 12-man legal team contested the very idea of his impeachment.

They called the two articles of impeachment -- approved largely along party lines in the Democratic-controlled House -- the product of "a rigged process" and "constitutionally deficient on their face" because they involved no violation of established law.

That team, which has recruited high-profile lawyers such as Kenneth Starr, who tried to bring down Clinton over his affair with Monica Lewinsky, said in the brief: "The Senate should reject the Articles of Impeachment and acquit the president immediately."

"President Trump abused the power of his office to solicit foreign interference in our elections for his own personal political gain, thereby jeopardizing our national security, the integrity of our elections, and our democracy," the House managers said Saturday in a memorandum.

They said the president's behaviour "is the Framers' worst nightmare," referring to the authors of the US Constitution, and that Trump deserves to be removed from office.

But Trump looks almost certain to be acquitted because of the 53-47 Republican majority in the Senate.

He will be abroad as his trial opens. Trump arrived Tuesday in snowy Davos, Switzerland, ahead of his scheduled speech to the World Economic Forum.

How long the trial will last is up in the air.

The first order of business Tuesday will be to set the rules, such as how long they will hear the arguments of the House managers, or prosecutors; how long they will hear the defence; the time allotted for questions, submitted by the senators but read by Roberts; and whether they will call witnesses or seek other evidence.

Senate Majority Leader Mitch McConnell late Monday proposed rules calling for each side to have 24 hours over two days to present their arguments. That makes for long trial days stretching late into the night but is a significantly quicker pace than in Bill Clinton's impeachment trial in 1999. The chamber will debate and vote on the proposed rules Tuesday.

Senate Democratic leader Chuck Schumer said McConnell is rushing the trial and also making it harder for witnesses and documents to be presented.

"On something as important as impeachment, Senator McConnell's resolution is nothing short of a national disgrace," Schumer said in a statement.

The Democrats want key Trump administration officials to testify, such as acting chief of staff Mick Mulvaney and former national security adviser John Bolton, in the belief that they know a lot about Trump's dealings with Ukraine. Bolton has said he is willing to testify if subpoenaed.

The White House has said it expects the trial to be over in two weeks. Clinton's trial lasted five weeks.

McConnell has said he won't consider the witness issue until after the arguments and questioning take place, and his majority means he will likely prevail.