Washington, DC - Senate Majority Leader Mitch McConnell said he has the backing of enough Republicans to begin a trial of President Donald Trump's impeachment as soon as the House sends articles to the Senate.

Under McConnell's plan, no decision would be made on whether to call witnesses or seek documents until after opening arguments in the trial are presented by members of the House of Representatives and the president's defence team.

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"We have the votes once the impeachment trial has begun to pass a resolution essentially the same, very similar to the 100-to-nothing vote in the Clinton trial, which sets up as you may recall what could best be described as maybe a 'phase one' which would include obviously arguments from the prosecution, arguments from the defence, and then written questions," McConnell said, referring to the impeachment trial of former President Bill Clinton.

"At that point during the Clinton trial, the appropriateness of calling witnesses was addressed," McConnell said. "Obviously, that is one of the most contentious parts of one of these proceedings and that will be addressed at that time and not before the trial begins."

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Democrats have demanded McConnell agree to call several Trump White House aides including acting Chief of Staff Mick Mulvaney and former National Security Adviser John Bolton to testify before the Senate.

"We don't know what all the evidence will say," Schumer said in remarks to the Senate earlier on Tuesday.

"It may exculpate the president. It may incriminate him. We only want a trial that examines all the facts and let the chips fall will they may," Schumer said.

House Speaker Nancy Pelosi has not yet sent articles of impeachment against Trump to the Senate, temporarily delaying the start of a Senate trial. She could send the articles to the Senate as soon as this week.

Witnesses?

The House voted on December 17 to impeach Trump for abuse of power and obstruction of Congress related to his dealings with Ukraine.

"All I have seen from Mitch McConnell over the past five years is that he is going to do everything to protect his members and protect the president," said Democratic Senator Chris Murphy.

"I don't think he is going to bring witnesses. This is all a whitewash to eventually bring this trial to conclusion without any production of witnesses or documents," Murphy told reporters.

Republicans cited the precedent set by the Senate in the impeachment trial of Clinton when decisions on calling witnesses were deferred until after opening arguments were made.

"We should only consider what the House sends us," said Senator Chuck Grassley, a Republican.

"We shouldn't do the House's work," Grassley told reporters in the Capitol on Tuesday.

Trump was impeached in December for abuse of power and obstruction of Congress [File: Evan Vucci/AP Photo]

Bolton issued a statement on Monday saying he would be willing to testify if the Senate issues him a subpoena.

Representative Adam Schiff, the Democrat who led the impeachment inquiry in the House, told reporters the president's refusal to allow Bolton to appear before House investigators suggests "his testimony would only be further incriminating."

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A trial in the Senate is expected to end with an acquittal. The chamber is made up of 53 Republicans, 45 Democrats and two independents who caucus with the Democrats. At least 20 Republicans would have to vote with all Democrats and two independents to remove the president from office.

The impeachment inquiry of Trump centred on a July 25 phone call between the US president and his Ukrainian counterpart. Democrats allege Trump abused his power of office by organising a pressure campaign to exhort a promise from Ukrainian President Volodymyr Zelenskyy to announce investigations into Trump political rival and former Vice President Joe Biden and a debunked conspiracy theory that Ukraine, not Russia, interfered in the 2016 US elections.

On the July call, Trump urged Zelenskyy to open an investigation into Biden, who is also a 2020 Democratic presidential frontrunner, and his son, Hunter, who had served on the board of a Ukrainian gas company. There has been no evidence of wrongdoing by the Bidens.

At the time of the call, the Trump administration was withholding nearly $400m in Congress-approved military assistance from Ukraine.