The president has called it “a charade”.

His lawyers have termed it part of a broader “illegitimate partisan effort to take him down” by Democrats.

But whether he likes it or not, Donald Trump has been impeached by the House of Representatives, and the “trial” of those articles is about to be heard in the Senate.

Much of what will play out over the coming days and even weeks, remains unclear.

Late on Monday, on the very eve of the start of proceedings, the Republican leader in the Senate, Mitch McConnell, issued a set of rules which he said will govern how the proceedings take place. Yet even at this stage, as many things are fluid as they are fixed in stone.

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

What will happen on Tuesday January 21?

The impeachment process trial resumes at 1pm EST (6pm GMT) on Tuesday. It is likely that very soon after it convenes, the chamber will be asked to vote on a resolution to govern the proceedings. Late on Monday, Mr McConnell proposed a condensed, two-day calendar for each side to give opening arguments. It also pushes off any votes on witnesses until later in the process, rather than up front, as Democrats demanded.

According to Mr McConnell, after the four days of opening arguments – two days per side – senators will be allowed up to 16 hours for questions to the prosecution and defence, followed by four hours of debate.

Will there be any witnesses?

Democrats have been pushing to be permitted to call several witnesses, among them former and serving White House officials, to give evidence. While some Republicans in the Senate have not ruled out supporting such a move, it has now become clear a decision regarding those witnesses will not be decided on Tuesday unless Democrats are some how able to force the issue. On Monday the Democratic leader in the Senate, Chuck Schumer, termed the Republican plan a “cover-up”.

When will the prosecution and defence be heard?

Once the rules have been adopted, Democratic House “managers” who form the prosecution team will begin to present their case against the president. It is unclear whether that will start on Tuesday or the next day. One report said the first evidence would likely be aired on Wednesday. When the House managers have finished, the president’s team will respond with its opening arguments.

Could the White House seek to have the two articles dismissed?

Reuters quoted a senior aide to the Republican leadership as saying lawyers for Mr Trump could move early in the proceedings to ask senators to dismiss all charges. It said such a motion that would likely fall short of the support needed to succeed.

Rep. Adam Schiff reads articles of impeachment in Senate

So what about witnesses such as John Bolton?

CNN pointed out Mr Connell’s resolution also said after presentations from the impeachment managers and the president’s legal team and then 16 hours of questions from senators, the Senate will consider “the question of whether it shall be in order to consider and debate under the impeachment rules any motion to subpoena witnesses or documents”.

When will the senators actually vote?

It had been expected that because of the ongoing wrangling, no such vote would take place before the end of January or beginning of February. If Mr McConnell gets his way, that process could be quicker than anticipated.

Are any Republicans likely to vote to impeach Trump?

At this point, no. In the unlikely event the Senate voted to impeach the president, he would be replaced by Mike Pence.

What else is on the president’s agenda?

On 4 February, a day after Democrats hold their first primary ballot of the 2020 political cycle in Iowa, Mr Trump is scheduled to deliver the annual State of the Union address to a joint session of Congress.

How does all of this impact the president’s re-election chances?

Assuming he is cleared by the Senate, the president will continue to portray himself as a victim of Democrats’ ill intentions. It will likely harden support among his base. What remains unclear is whether it helps earn him new supporters. Most likely, by November 2020 so many other things will have played out that impeachment is far from the minds of most voters.