The Senate will soon be seated as a court of impeachment to hear charges that Donald Trump abused his office by withholding $391 million in military aid to Ukraine in order to force that country’s president to announce investigations into former Vice President Joe Biden and a discredited conspiracy theory positing that Ukraine – not Russia – interfered in the 2016 election.

He is also accused of obstructing Congress’s investigation into whether he’d done so by ordering aides to neither testify nor turn over documents to impeachment investigators.

The start of the trial comes after a month-long delay initiated by House Speaker Nancy Pelosi, who took the unprecedented but constitutionally permissible step of postponing handing off the articles of impeachment to the Senate after the upper chamber’s Majority Leader Mitch McConnell said he would be coordinating the conduct of Mr Trump’s trial with the White House Counsel’s office and the president’s other lawyers.

Ms Pelosi said at the time that the delay was necessary in order to obtain assurances that the rules Mr McConnell would put forth for Mr Trump’s trial would provide for testimony by a number of witnesses who Mr Trump had ordered to not cooperate with the House’s impeachment inquiry.

Those potential witnesses include White House Acting Chief of Staff Mick Mulvaney, Mulvaney aide Robert Blair, Office of Management and Budget official Michael Duffey, and Ambassador John Bolton, Mr Trump’s former national security adviser.

All the president's lawyers: The team fighting Trump's impeachment Show all 6 1 /6 All the president's lawyers: The team fighting Trump's impeachment All the president's lawyers: The team fighting Trump's impeachment Alan Dershowitz Dershowitz is a controversial American lawyer best known for the high-profile clients he has successfully defended. Those clients have included OJ Simpson, Jeffrey Epstein and Harvey Weinstein. One longtime Harvard Law associated told the New Yorker Dershowitz "revels in taking positions that ultimately are not just controversial but pretty close to indefensible." Getty All the president's lawyers: The team fighting Trump's impeachment Ken Starr Starr became a household name in the 1990s as the independent counsel who led the investigation that led to Bill Clinton's impeachment. That investigation began as a look into a real estate scandal known as Whitewater, and eventually led to impeachment after Mr Clinton lied under oath about having an affair with White House intern Monica Lewinsky. AP All the president's lawyers: The team fighting Trump's impeachment Jay Sekulow Sekulow is the president's longtime personal attorney, and, now, personal lawyer in the White House. He has been accused by former Rudy Giuliani associate Lev Parnas of being "in the loop" during the Ukraine scandal. Getty All the president's lawyers: The team fighting Trump's impeachment Pam Bondi Bondi is the former attorney general in Florida, and a longtime backer of the president's. She made a name for herself in Florida for taking hyper partisan stances on issues, and her penchant for publicity. She is likely to be a prominent public-facing figure during the trial. AFP/Getty All the president's lawyers: The team fighting Trump's impeachment Pat Cipollone Cipollone is the White House counsel, and leading the president's defence team. Getty All the president's lawyers: The team fighting Trump's impeachment Rudy Giuliani While not officially named as one of the president's impeachment lawyers, it is hard to ignore Giuliani's outsized role in this process. The former mayor of New York has been making headlines for months as he defends his client, and for his apparent role in the effort to compel Ukraine to launch the investigation into Joe Biden. We'll see how he figures in the actual trial, which he has said he would like to be a part of. Reuters

Mr McConnell has so far refused to make any such assurances, and has instead declared that the conduct of Mr Trump’s trial would closely track the procedures used during the 1999 impeachment trial of then-president Bill Clinton.

During Mr Clinton’s trial, all 100 senators agreed to not make any decision on whether or not to call witnesses until they’d heard opening arguments from the House “managers” – members appointed to prosecute the case against the president – and from the president’s attorneys. Senators later voted to compel videotaped testimony from several witnesses, including former White House intern Monica Lewinsky.

While many Republicans criticised Ms Pelosi’s decision to withhold the articles as an abuse of power which some Trump allies said meant the president had not yet been impeached, the 28 days between the House’s vote to impeach Mr Trump and the transmission of the articles of impeachment to the Senate have seen breathtaking new evidence emerge and one significant witness declare that he’d testify if called.

What we’ve learned since the House voted to impeach the president.

Since the House’s impeachment inquiry began on 24 September, Republicans in both the House and Senate have complained that Democrats lack firsthand testimony and documentary evidence – the same evidence that Mr Trump has ordered executive branch officials not to provide to impeachment investigators.

While Mr Trump’s subordinates have stonewalled Congress, outside actors have been able to use the courts and the US Freedom of Information Act to uncover new evidence which sheds more light on both the details of how funding to Ukraine was withheld, by whom, and their motivations for withholding it.

Just four days after the House voted to impeach the president, White House emails released as part of a lawsuit pursuant to FOIA revealed how the Office of Management and Budget ordered the US Defence Department to stop releasing military aid to Ukraine just 91 minutes after the president’s July phone call with Ukrainian President Volodymyr Zelensky, during which Mr Trump responded to his counterpart’s request to purchase more US-made Javelin anti-tank missiles by telling him: “I would like you to do us a favour, though” by investigating the 2016 election conspiracy theory and the family of Mr Biden.

He added that Mr Zelensky should contact his personal attorney Rudy Giuliani for assistance.

On 2 January, the legal blog Just Security published unredacted versions of some of those emails which showed how OMB officials told Pentagon officials that the order to withhold Ukraine’s military aid came at the “clear direction” of Mr Trump, and that OMB officials ignored concerns that continuing to withhold “obligation” of the funds would prevent them from being spent on Ukraine’s defence at all.

Two days later, Mr Bolton thrust Democrats’ demand for witnesses back into the spotlight when he declared that he would – if subpoenaed – testify against his former boss after a judge’s ruling in a similar case left him no way to legally decline a Senate subpoena. An attorney for Mr Bolton has said his client, who resigned from Mr Trump’s administration on 10 September, has personal knowledge of information relevant to the inquiry.

It is thought that Mr Bolton’s testimony could be highly damaging to Mr Trump’s defence. During the House Intelligence Committee’s public impeachment hearings, Bolton aide Dr Fiona Hill testified that her boss, a Yale-educated attorney, advised her to speak to White House lawyers after a meeting with Ukrainian officials, during which Ambassador to the European Union Gordon Sondland said that Mr Zelensky could secure a public Oval Office meeting with Mr Trump by announcing the investigations which the president desired.

According to Dr Hill, Mr Bolton called the arrangement a “drug deal” and said he wanted no involvement with it.

Mitch McConnell announces Trump impeachment trial will begin 21 January

Last minute evidence dump casts puts spotlight back on Trump lawyer

But even after Mr Bolton’s testimony became a possibility, the biggest bombshells were yet to come. Just one day before the House would send the articles of impeachment to the Senate, House Democrats released yet more evidence gathered from Lev Parnas, an associate of Trump attorney Rudy Giuliani who has been indicted for violating campaign finance laws.

A handwritten note in his handwriting shows that the goal of Mr Parnas, who was working for Mr Giuliani on Mr Trump’s behalf, was to get Mr Zelensky, then Ukraine’s president-elect, to announce an investigation into Mr Biden.

While Mr Trump’s Republican defenders have claimed that the president’s decision to withhold aid to Ukraine and request the announcement of an investigation into Mr Biden was made pursuant to the president’s foreign policy priorities, a letter from Mr Giuliani to Mr Zelensky shows the former New York City mayor knew he was acting as Mr Trump’s personal attorney, for Mr Trump’s personal benefit.

Fiona Hill: 'What we're seeing here as a result of all of these narratives is this is exactly what the Russian government was hoping for'

The letter, dated 10 May 2019, is printed on stationery belonging to Mr Giulani and undermines the defence offered by Trump allies who say the president was using him as a “back channel” to advance legitimate foreign policy interests.

It says: “I am private counsel to President Donald J Trump. Just to be precise, I represent him as a private citizen, not as President of the United States”.

Mr Giuliani won’t be participating in Mr Trump’s trial defence, but a senior administration official said the White House lawyers who, along with Trump attorney Jay Sekulow, will take the lead are confident that they’ll be able to easily make a case to acquit the president.

“These are the weakest articles of impeachment that have ever been passed,” the official said, adding that the House’s investigation had found “no violation of any law” and that “the idea that it is obstruction of Congress for the president to assert constitutionally protected privilege is absurd”.

“We’re happy that we’re going to have a chance to vindicate the president ... so the country can move on,” the official said.

What’s next?

Now that the House “managers” have delivered the articles of impeachment to the Senate, that body will convene on Tuesday 21 January as a “court of impeachment” presided over by John Roberts, the Chief Justice of the United States.

Unlike normal Senate proceedings, during which few senators are present on the senate floor, all 100 members will be required to be seated at their desks to hear testimony. And in what will be a painful twist for a group of normally loquacious politicians, they will have to remain silent and will only be able to submit questions or motions in written form.

Current rules governing impeachment state that the body will convene each day at 1pm, but that could change if enough senators vote to alter the schedule.

It won’t only be senators whose routines will be altered by impeachment. Reporters who cover the Senate will be subject to a whole host of restrictions for the duration of the trial.

Instead of being able to question senators as they move about the Capitol, reporters will be confined to the same sort of pen which Trump campaign events use to prevent journalists from speaking with rally attendees. The only time reporters will be permitted to question senators will be if the politicians themselves choose to approach the “pen” area.

The restrictions are the brainchild of Senate Rules Committee Chair Roy Blunt, a Missouri Republican and close Trump ally.

Donald Trump says John Bolton 'would would know nothing about what we're talking about' ahead of impeachment trial

In a statement, the Standing Committee of Correspondents, which represents journalists accredited to cover the US Congress, said Mr Blunt and Capitol security officials rejected the committee’s suggestions on how to manage the large influx of press expected to cover the trial “without an explanation of how the restrictions contribute to safety rather than simply limit coverage of the trial”.