It is a rare moment in American history. Donald Trump, the 45th President of the United States, is expected to become only the third president to be impeached.

It has been an acrimonious process and the vote, much like the hearings, will be divided along party lines.

That unrelenting political division was on show in the final hours.

Just as the House Rules Committee was working out how the day would play out, President Trump was releasing a six-page excoriating letter to the Speaker of the House Nancy Pelosi.

In it, Mr Trump not only claimed that “more due process was afforded to those accused in the Salem Witch Trials” (which led to 19 people being hung as witches in the 17th century), but also accused Mrs Pelosi of portraying a “false display of solemnity”.

It was a furious lament from a man who claims the Democrats have been out to get him from the start.

They are seeking to remove him from office for pressing Ukraine to investigate political rival Joe Biden earlier this year.

More from Donald Trump

He has accused the Democrats of pursuing an “illegal, partisan attempted coup” and declaring war on American democracy.

But the framework to decide his fate is now set.

First they’ll have a debate to ultimately set-up the actual debate on the articles of impeachment. The latter should take place in the morning Washington time and could take around four hours.

After that comes the key vote – possibly in the early evening.

Even getting both sides to agree on the parameters of the day was a divisive affair.

Democrat Jamie Raskin said: “We present you not just with high crimes and misdemeanours – but a constitutional crime in progress, up to this very minute.”

This is how impeachment works in two minutes
This is how impeachment works

Republican Doug Collins fired back, casting their work as mere political fantasy: “We are more Alice in Wonderland than we are House of Representatives.”

The president faces two charges: abuse of power and obstruction of congress.

The Democrats only need a simple majority in the House of Representatives, which they control, to impeach Mr Trump.

They should reach that with ease, barring perhaps two defections. But not a single Republican is expected to support impeachment – not a good look for a party who preached the need to impeach on a bipartisan basis for much of 2019.

After that, it goes to trial.

The Republican-controlled Senate will very likely acquit President Trump. But there’s unlikely to be much co-operation between the two parties.

Senate majority leader Mitch McConnell has made it clear they won’t be entertaining the Democrats’ request for four White House witnesses to testify.

Speaker of the House Nancy Pelosi (C), flanked by House Judiciary Chairman Jerry Nadler, (L) (D-NY) and House Committee on Oversight and Reform Chairwoman Carolyn Maloney (R) (D-NY), announces articles of impeachment against US President Donald Trump during a press conference at the US Capitol in Washington, DC, December 10, 2019. - Democrats listed abuse of power and obstruction of Congress. (Photo by SAUL LOEB / AFP) (Photo by SAUL LOEB/AFP via Getty Images)
Image: Mr Trump has written a six-page letter to the Speaker of the House Nancy Pelosi

“I’m not an impartial juror. This is a political process. There’s not anything judicial about it,” Mr McConnell told reporters.

This is the first time that foreign policy and the role of a foreign power has been at the heart of an impeachment investigation.

What also marks it out is how little the inquiry has changed voters’ opinions.

A Washington Post-ABC News poll showed 49% of Americans believe Mr Trump should be impeached and removed from office. Some 46% say he should not.

Those numbers are almost exactly the same as before the process started.

It is a day for the history books. Impeachment is a stain on any president’s legacy, but it need not be disqualifying. Bill Clinton’s approval ratings soared after his process. Trump could well win 2020 off the back of his.

Impeachment is a double-edged sword – rarely used and for good reason. It’s designed as a check on power, but it nullifies the will of voters.

For Democrats it is indisputably a political risk. They insist it’s one they are duty bound to take and one they carry out with heavy hearts.