Washington, DC - Opening arguments in the United States Senate impeachment trial of President Donald Trump are set to begin on Wednesday after Democrats and Republicans waged an overnight power struggle over witnesses and evidence.

Led by Democratic congressman Adam Schiff, the House's prosecution team will rely on evidence developed in the House's impeachment inquiry last year.

"We're ready to present our case, ready to call witnesses, ready to see the documents," Schiff said in arguments before the Senate on Tuesday.

More:

Trump was impeached on December 18 for abuse of power and obstruction of Congress.

Schiff and fellow House managers plan to use video clips from 12 witnesses who appeared in House hearings, interspersed with text messages and emails, to build a public timeline of Trump's pressure campaign on Ukraine.

Democrats accuse Trump of withholding nearly $400m in US security aid in an attempt to coerce Ukraine President Volodymyr Zelenskyy to open a sham investigation into Democratic presidential contender Joe Biden.

House managers are expected to argue Trump imperilled US national security by illegally delaying the aid to Ukraine and, in seeking the Biden investigation, Trump solicited interference by Ukraine in the upcoming US presidential election.

Democrats, citing witness testimony, also say Trump withheld a White House meeting Zelenskyy was seeking until the Ukrainian government announced an investigation of Biden.

Under US law, the president cannot block the distribution of congressionally approved funds without an explanation to Congress. The Government Accountability Office said the White House broke federal budget law by blocking the funds.

Further, it is illegal for Americans to seek foreign contributions of any kind to US political campaigns.

At the centre of the impeachment inquiry was a July 25 telephone call between Trump and Zelenskyy, in which Zelenskyy asked to buy US weapons and Trump responded: "Do us a favour, though" by looking into the Bidens. There is no evidence former Vice President Biden, or his son Hunter Biden, who served on the board of Ukrainian gas company Burisma, did anything wrong.

Trump attends a bilateral meeting during the World Economic Forum (WEF) annual meeting in Davos [Fabrice Coffrini/AFP]

Speaking at the World Economic Forum in Davos, Trump repeated his claims that the call with Zelenskyy was "perfect", that there was no pressure applied to Ukraine. He has called the impeachment a "hoax" and a "witch-hunt".

Democrats press for witnesses

The trial took a contentious turn on Tuesday on its second day as Republicans repeatedly voted to deny Democratic requests for witnesses and documents from the White House.

Democrats proposed the Senate issue subpoenas to the White House, the State Department and the Office of Management and Budget for documents and emails.

Democrats also asked to subpoena former National Security Adviser John Bolton and acting White House Chief of Staff Mick Mulvaney, among others, to testify in the Senate trial.

Trump in Davos said Bolton should not be allowed to testify because of national security concerns.

"The problem with John is that it's a national security problem," Trump said. "He knows some of my views" on foreign leaders, Trump said.

In this image from video, Senators vote on approving the rules for the impeachment trial against President Donald Trump in the Senate [Senate Television/AP Photo]

Republicans defeated 11 Democratic proposals to subpoena documents and people in a series of votes along 53-47 party lines that concluded in the early hours on Wednesday.

House managers will have 24 hours of floor time over three days to make their case for Trump's impeachment and removal from office.

The president's lawyers will then have 24 hours over three days to mount a defence.

That is a change from the initial two days of arguments for each side that Senate Majority Leader Mitch McConnell had proposed. He made the handwritten changes to the House rules resolution abruptly, following objections from Democrats and some Republicans.

'Nothing wrong'

Trump's defence team, led by White House Counsel Pat Cipollone, gave a preview of what its arguments might look like on Tuesday.

Cipollone repeatedly argued the charges against the president did not come close to meeting the US Constitution's standard for impeachment.

"The only conclusion will be that the president has done absolutely nothing wrong," Cipollone said, as he argued in favour of McConnell's proposal to decide on whether to allow further witnesses or documents later in the trial.

"There is absolutely no case," he said.

White House counsel Pat Cipollone speaks during opening arguments in the US Senate impeachment trial of Donald Trump [US Senate TV/Reuters]

After an expected six days of arguments, senators will have 16 hours to submit written questions to the House managers and Trump's defence, after which the Senate will revisit whether to call witnesses.

A majority - 57 percent - of Americans say that House managers should be able to introduce new evidence in the Senate impeachment trial, according to a new poll by Monmouth University.

More than half of the American public - 53 percent - approve of the House of Representatives' decision to impeach Trump, while 46 percent disapprove, according to the Monmouth poll.

The survey of 903 adults was taken between January 16 and 20 and had a 3.3 percentage-point margin of confidence.



With Republicans standing together in defence of Trump, a guilty verdict in the trial is unlikely.

"The end of this proceeding is going to be an acquittal," Senator Ted Cruz told reporters on Tuesday.

Trump is only the third US president to be impeached. No president has ever been removed from office by the Senate.