The US Senate has voted to approve the rules governing the impeachment trial of President Donald Trump, including delaying a debate over whether to call witnesses until the middle of the trial.

Key points: The debate lasted well past midnight as both sides voted along party lines

The debate lasted well past midnight as both sides voted along party lines Republicans turned back Democratic amendments to subpoena documents from the White House

Republicans turned back Democratic amendments to subpoena documents from the White House More votes are expected to reject key witnesses with a front-row seat to Mr Trump's actions

With Republicans banding together, the Senate voted 53-47 to adopt the trial plan, which allows opening arguments from House members prosecuting the case.

Republicans abruptly abandoned plans to cram opening arguments into two late-night sessions and Democrats argued for more witnesses to expose Mr Trump's "trifecta" of offences.

A marathon session of nearly 13 hours featured a setback for Republican Senate leader Mitch McConnell and the President's legal team, exposing a crack in the GOP ranks and the growing political unease over the historic impeachment proceedings unfolding amid a watchful public in an election year.

Senators remained as the clock passed 10:30pm (local time), while Democrats pursued what may be their only chance to force senators to vote on hearing new testimony.

However, Republicans turned back Democratic amendments to subpoena documents from the White House, State Department, Defence Department and budget office, with more votes expected to reject key witnesses with a front-row seat to Mr Trump's actions.

By the same 53-47 party-line, senators turned aside the Democrats' request to subpoena acting White House chief of staff Mick Mulvaney.

As the hours mounted, Mr McConnell offered Democrats a deal to stack the votes more quickly, but Senate Minority Leader Chuck Schumer countered that voting could resume the next day. No deal was reached.

"It's time to start with this trial," said White House counsel Pat Cipollone, the President's lead lawyer, voicing impatience as the proceedings opened in public after weeks of delay.

Mr Cipollone scoffed that the House charges against Mr Trump were "ridiculous," insisting the President "has done absolutely nothing wrong".

The impeachment trial is also testing whether Mr Trump's actions toward Ukraine warrants removal. ( AP: Manuel Balce Ceneta )

The White House legal team did not dispute Mr Trump's actions, when he called Ukraine and asked for a "favour", which was to investigate Democrat Joe Biden as the US was withholding military aid the ally desperately needed as it faced off with hostile Russia on its border.

Democrat Adam Schiff opened for the prosecution saying America's founders added the remedy of impeachment in the Constitution with "precisely this type of conduct in mind — conduct that abuses the power of office for a personal benefit, that undermines our national security, and that invites foreign interference in the democratic process of an election".

Adam Schiff said that the Constitution included the impeachment option to deal with abuses of power. ( AP )

McConnell cancels rushed plans after concerns from Republicans

Mr McConnell made the adjustment after encountering resistance from Republicans during a closed-door lunch meeting.

Senators worried about the public reaction to cramming the 24 hours of opening arguments from each side into just two days.

Some senators argued that the two-day limit would have helped Democrats cast Republicans as squeezing testimony through in the dead of night.

The White House wanted a session kept to a shorter period to both expedite the trial and shift more of the proceedings into late night, according to a person familiar with the matter but unauthorised to discuss it in public.

Mr Schumer said Republican senators "felt the heat" and it "shows that they can make other changes and that we can get documents and witnesses".

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Mr Schumer offered the first of several amendments to the rules — to issue a subpoena for to the White House for "all documents, communications and other records" relating to the Ukraine matter.

It was rejected on a party-line vote, as were the others.

This artist sketch depicts White House counsel Pat Cipollone speaking in the Senate chamber during the impeachment trial. ( AP: Dana Verkouteren via AP )

Mr Trump's legal team argued that in seeking new evidence the House was bringing a half-baked case that would be thrown out in any other court of law.

The White House had instructed officials not to testify in the House inquiry, and refused to turn over witnesses or documents, citing what it says is precedence in defiance of congressional subpoenas.

The ambassadors and national security officials who did appear before the House delivered often striking testimony, highlights that were displayed on television screens during the Senate proceeding.

Democrat Val Demings, a former police chief, said she never saw anyone take "such extreme steps to hide evidence''.

No President has ever been removed from office by the Senate.

With its 53-47 Republican majority, the Senate is not expected to mount the two-thirds vote needed for conviction.

AP