The US Senate plunged into President Donald Trump's impeachment trial on Tuesday with Republicans abruptly abandoning plans to cram opening arguments into two late-night sessions and Democrats arguing for more witnesses to expose Trump's 'trifecta' of offenses.

The turn of events was 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.

Chief Justice John Roberts gaveled open the session, with House prosecutors on one side, Trump's team on the other, in the well of the Senate, as senators sat silently at their desks, under oath to do 'impartial justice.' No cellphones or other electronics were allowed.

Opening day stretched deep into the night. Senators remained as the clock passed 10.30pm, 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, defense department and budget office, with more votes expected rejecting key witnesses with a front-row seat to 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, McConnell offered Democrats a deal to stack the votes more quickly, but Senate Minority Leader Chuck Schumer countered that voting could resume Wednesday. No deal was reached.

'It's not our job to make it easy for you,' Rep. 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, 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, for a moment swelling 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 Trump's most ardent House allies lining the back rows, the day quickly took on the cadence of a trial proceeding 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.

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

The White House legal team did not dispute Trump's actions, when he called Ukraine and asked for a 'favor,' which was to investigate Democrat Joe Biden as the U.S. was withholding military aid the ally desperately needed as it faced off with hostile Russia on its border. But the lawyers insisted the President did nothing wrong. 'Absolutely no case,' Cipollone said.

Schiff, the California Democrat, 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.'

Said Schiff: 'It is the trifecta of constitutional misconduct justifying impeachment.''

The other lead lawyer on Trump's team, Jay Sekulow, retorted, 'I'll give you a trifecta,' outlining complaints over the House Democrats' impeachment inquiry process.

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

All four senators who are presidential candidates are off the campaign trail, seated as jurors. 'My focus is going to be on impeachment,' Sen. 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.

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.

Sen. Susan Collins of Maine and Sen. 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 squeezing testimony through in the dead of night.

Collins and Murkowski, who often ally to buck GOP leadership, sat side by side in the Senate through the day's proceedings.

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 unauthorized to discuss it in public.

'READ THE TRANSCRIPTS!' the President tweeted from overseas, at a global leaders conference in Davos, Switzerland.

That's the transcript of his phone call in which he asked new Ukrainian President Volodymyr Zelenskiy for 'a favor.' The Democrats cite that transcript as solid evidence against Trump, though he repeatedly describes it as 'perfect.'

A whistleblower's complaint led the House to impeach Trump last month on a charge of abuse of power for pushing Ukraine to investigate Democratic rival Biden and his son Hunter Biden as the White House was withholding military aid from Ukraine. Trump also was impeached on a second charge, of obstruction of Congress, in the House probe.

Schumer of New York said Republican senators 'felt the heat' and it 'shows that they can make other changes, and that we can get documents and witnesses.'

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.

Trump's legal team, absent its TV-showcase attorneys, Alan Dershowitz and Kenneth Starr who were not in the chamber, 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.

But Rep. Zoe Lofgren of California, one of the House managers and the first woman to argue for the prosecution in a presidential impeachment trial, said the House wasn't asking the Senate to do the job for them. 'The House is asking the Senate to do its job, to have a trial,' she said. 'Have you ever heard of a trial without evidence?''

The White House had instructed officials not to testify in the House inquiry, and refused to turn over witnesses or documents, citing what is 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.

At one point, Democrat Schiff displayed video of Trump himself suggesting there should be more witnesses testifying.

One by one, the House managers made the case, drawing on their own life experiences.

Rep. Val Demings, D-Fla., a former police chief, said she never saw anyone take 'such extreme steps to hide evidence.'' Rep. Jason Crow, a former Army Ranger who served in Iraq and Afghanistan, seemed to capture senators' attention when he told them near he knew the hour was late, but it was morning in Ukraine where soldiers were waking up to fight Russia, depending on US aid.

Legal scholars have long insisted the framers of the Constitution provided impeachment as a remedy for 'other high crimes and misdemeanors,' a particularly broad definition that doesn't mean simply specific criminal acts.

Democrats point in particular to a General Accountability Office report that found the White House violated federal law by stalling money to Ukraine that had been approved by Congress.

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.