Donald Trump’s impeachment trial has begun with Republicans abandoning plans to cram opening arguments into two late-night sessions and Democrats wanting more witnesses to expose the president’s alleged treble of offences.

The turn of events was a setback for Republican Senate leader Mitch McConnell and the president’s legal team, exposing a crack in GOP ranks and the growing political unease over the historic impeachment proceedings unfolding in an election year.

Chief Justice John Roberts opened the session, with House prosecutors on one side of the Senate and Mr Trump’s team on the other as senators sat silently at their desks, under oath to conduct “impartial justice”. No cellphones or other electronics were allowed.

Supreme Court Chief Justice John Roberts at the start of the impeachment trial. Credit: Senate Television via AP

Opening day stretched deep into night. Senators remained as the clock passed 10:30pm as 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 rejecting 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 on Wednesday, and no deal was reached.

  • Donald Trump's impeachment trial explained

“It’s not our job to make it easy for you,” Representative Adam Schiff, the chairman of the House Intelligence Committee leading the prosecution, told the Senate. “Our job is to make it hard to deprive the American people of a fair trial.”

Earlier, Mr McConnell stunned senators and delayed the start of proceedings with his decision to back off some of his proposed rules. Republicans were said to be concerned over the political optics of “dark of night” sessions.

Instead, 24 hours of opening arguments for each side will be spread over three days, temporarily swelling the Democrats’ momentum as they push to break the standoff over calling new witnesses.

As the visitors’ gallery filled with guests, actress-and-activist Alyssa Milano among them, and Mr Trump’s most ardent House allies lining the back rows, the day quickly took on the cadence of a trial over whether the president’s actions toward Ukraine warranted removal from office.

“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.

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 its ally desperately needed as it faced off with hostile Russia on its border.

But Mr Cipollone scoffed that the House charges against Mr Trump were “ridiculous”, insisting the president had done “absolutely nothing wrong”.

Mr Schiff, a California Democrat, opened for the prosecution saying America’s founders had 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”.

He added: “It is the trifecta of constitutional misconduct justifying impeachment.”

The other lead lawyer on Mr Trump’s team, Jay Sekulow, retorted by outlining complaints over the House Democrats’ impeachment inquiry process.

The impeachment trial, unfolding in an election year, will test whether Mr Trump’s actions toward Ukraine warrant removal at the same time that voters are forming their own verdict on his White House.

An artist’s sketch depicting Senate Minority Leader Chuck Schumer speaking in the trial. Credit: Dana Verkouteren/AP

All four senators who are presidential candidates are off the campaign trail, seated as jurors.

“My focus is going to be on impeachment,” Senator Bernie Sanders, the Vermont independent, told reporters.

The day began as a debate over rules, and it was only when the clerk started reading the dry language of the resolution that the hand-written changes became apparent.

Mr McConnell made the adjustment after encountering resistance from Republicans during a closed-door lunch meeting. Senators expressed concern about the public reaction to cramming the 24 hours of opening arguments from each side into just two days.

Senator Susan Collins of Maine and Senator Lisa Murkowksi of Alaska, along with a substantial number of other Republicans, wanted to make the changes, according to people familiar with the situation. Some senators argued that the two-day limit would have helped Democrats cast Republicans as having rushed testimony through in the dead of night.

The turnaround was a swift lesson as White House wishes run into the reality of the Senate. 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.

“READ THE TRANSCRIPTS!” the president tweeted from overseas, at a global leaders conference in Davos, Switzerland.

He referred to the transcript of his phone call, in which he asked new Ukrainian President Volodymyr Zelenskiy for “a favour”. The Democrats cite that transcript as solid evidence against Mr Trump, though he has repeatedly described it as “perfect”.

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”.

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.

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.