After more than 12 hours of debate and several failed Democratic bids for amendments calling for documents and witnesses, the US Senate approved a resolution along party lines outlining the rules that will govern the impeachment proceedings of President Donald Trump.

With Republicans banding together, the Senate early on Wednesday voted 53-47 to adopt the trial plan, which allows opening arguments from House lawmakers prosecuting the case to begin later in the day.

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The trial started in earnest on Tuesday with Senate Majority Leader Mitch McConnell abruptly abandoning plans to cram opening arguments into two late-night sessions for each side.

Instead, managers from the House of Representatives and Trump's defence lawyers will now each have three days for 24 hours of opening arguments.

McConnell's handwritten, last-minute changes stunned fellow senators and delayed the start of the proceedings. He acted after protests from senators, including fellow Republicans, who were concerned about the optics of middle of the night sessions.

It was a dramatic setback for the Republican leader and the president's legal team, exposing a crack within the party's ranks and the political unease over the historic impeachment proceedings unfolding amid a watchful public in an election year.

Supreme Court Chief Justice John Roberts presides over the start of the Senate impeachment trial of President Donald Trump as the House impeachment managers sit to one side and the president's legal team sits on the other side in the Senate Chamber [US Senate TV/Reuters]

Rare event

Tuesday's session marked an historic day for the deeply divided country. Trump is only the third president in the history of the United States to face an impeachment trial.

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

The day swiftly took on the cadence of a trial proceeding over whether the president's actions towards Ukraine warranted removal from office.

With all 100 senators seated at their assigned desks in the Senate chamber and the Supreme Court chief justice presiding, most senators appeared to be listening carefully with serious looks on their faces. Some took notes. There were very few smiles.

Democrats put forth 11 amendments for subpoenas for documents and witnesses. One-by-one those amendments were debated and then blocked by Senate Republicans holding the 53-47 majority.

Under the rules package passed on Wednesday, evidence from the Democratic House's impeachment hearings will be included in the record - a Democratic demand. Opening arguments will be followed by 16 hours of questions and answers from senators before four hours of debate.

Preview of arguments ahead

Trump was impeached on December 18 for abuse of power related to his dealings with Ukraine and obstruction of Congress for refusing to participate in the impeachment investigation.

House Democrats accuse the president of abusing his power in office by orchestrating a pressure campaign to get Ukraine to investigate former Vice President Joe Biden, a leading Democratic political rival, as well as launch a probe into a debunked conspiracy theory that Ukraine, not Russia, meddled in the 2016 presidential elections.

The House impeachment investigation centred on a whistle-blower complaint that detailed concerns about a phone call in which Trump asked Ukrainian President Volodymyr Zelenskyy for the investigation into Biden and his son, Hunter, who had served on the board of a Ukrainian gas company.

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

Senate Democrats want to hear from at least four witnesses in the Senate trial - a demand Republicans have criticised. For now, the question of witnesses will be discussed after opening arguments and the question-answer session.

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

On Tuesday, Trump's defence team, led by White House Counsel Pat Cipollone, attacked the foundation of the charges against the Republican president and said Democrats had 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.

Democratic Representative Adam Schiff, who helped spearhead the House impeachment inquiry, summarised the charges against Trump and said the president had committed "constitutional misconduct justifying impeachment".

Schiff said that although the evidence against Trump was "already overwhelming", further witness testimony was necessary to show the full scope of the misconduct by the president and those around him.

Democratic Representative Adam Schiff, who helped spearhead the House impeachment inquiry, summarised the charges against Trump [US Senate TV/Reuters]

For his part, Trump has repeatedly denied any wrongdoing and labelled the impeachment a "hoax".

"READ THE TRANSCRIPTS!" the president tweeted from overseas miles away, as he returned to his hotel at a global leaders conference in Davos, Switzerland. He was referring to the White House summary of his call with Zelenskyy.

Democrats cite that transcript as solid evidence against Trump, though he repeatedly describes it as "perfect".

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

President Donald Trump delivers the opening remarks at the World Economic Forum [Evan Vucci/AP Photo]

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. He said his supporters would keep working "to defeat the most dangerous president in American history".

With Republicans having a majority, the Senate is not expected to mount the two-thirds voted needed for conviction. No president has ever been removed from office by the Senate.