Image source, BRENDAN SMIALOWSKI/AFP via Getty Images

Trump has dismissed the impeachment probe as a "witch hunt", a "scam" and a "hoax" - but he still has his eye on today's proceedings.

The president has tweeted or re-tweeted 45 times since House members began debate this morning.

With Democrats in control of the lower chamber of Congress, this impeachment vote is all but sure to pass.

So what happens next?

A trial in the Senate will follow, probably in early January.

The Constitution is admittedly vague when it comes to impeachment - simply mandating that the House has the “sole power of impeachment", bringing charges, while the Senate alone can try impeachments and convict a president of crimes.

There are general rules, external largely based on President Andrew Johnson's trial, but ultimately, Republican leader Mitch McConnell and his Democratic counterpart Chuck Schumer will have to determine the guidelines for evidence, witnesses, duration and arguments.

While Mr McConnell has the final say over the format as the Republican Senate leader, he could find his options limited if Democrats pressure moderate Republicans to vote with them on any changes to the rules. At any point, senators can call for votes on trial procedures that would need a simple majority to pass.

After lawmakers hear from both sides - House prosecutors and White House counsel - and any witnesses, they will be given a full day to deliberate before a vote on conviction is held.

Read the full story here.

Speaking before the House, Massachusetts Congressman Joseph Kennedy III addressed his two children, describing the day as "a moment that you'll read about in your history books".

And Kennedy has intimate knowledge of the political events that have shaped the US, as grandson of assassinated presidential candidate Robert Kennedy.

Among his three most famous family members - John, Robert and Edward - the Kennedys served a combined 66 years in the chamber.

Currently serving his fourth term in Congress, this younger Kennedy is seen as leader of the political dynasty's next generation.

"I don't yet know how they will tell the story of this era, but I want to tell you the story of this day," Kennedy said. "Today, justice won."

And sticking with approval ratings for a moment...

A poll released yesterday by the Military Times, external found half of active service members are displeased with the president.

However, these numbers are still higher than President Barack Obama's were in January 2017, according to the Times.

And as for impeachment? The Times found 47% backed the proceedings, while 46% opposed - much like the broader population.

Image source, Gallup

All signs point to impeachment in the Democratic-controlled house, but you might not know it looking at Trump's approval ratings.

The president now boasts a 45% job approval rating, according to the latest poll from Gallup, released this morning.

That's still lower than any president would want, but Trump's ratings have actually increased by six percentage points since the House launched an impeachment inquiry against him in the autumn, and they are just one point down from his all-time high of 46%

And in more bad news for Democrats, Gallup found that a whopping 89% of Republicans approve of Trump's performance on the job.

Only 8% of Democrats agree.

As we await the historic vote, here's a look back at what lawmakers on both sides of the aisle said during Democratic President Bill Clinton's impeachment.

Some of the grievances aired then may sound familiar.

Both Democrats and Republicans are now using similar lines to criticise the opposing party's response to the 2019 impeachment inquiry against President Trump.

As the BBC reported in 1998:

While Republicans insisted Mr Clinton's actions amounted to the "high crimes and misdemeanours", the definition of impeachment, Democrats claimed censure would be more appropriate.

On Friday, Vice-President Al Gore said there was more chance of a "meteor strike" than "the resignation of the president".

With a Republican majority in Congress, the vote was all but a foregone conclusion.

Congressional lawmakers are currently arguing over impeachment on the House floor.

Officially, they are meant to be debating the rules of the proceedings, and leave the actual substance of the articles of impeachment for the main debate later.

But this being Washington, they can't resist landing some punches on the enemy.

Republicans continue to blast the inquiry as partisan and rigged, while Democrats maintain that the president committed a crime and they are duty-bound to see this through.

One of Trump’s opponents in the 2016 Republican primary campaign was ex-HP executive Carly Fiorina, whom Trump famously criticised by mocking her appearance (“Look at that face. Would anyone vote for that?”).

Speaking to CNN this week, Fiorina endorsed the House impeachment vote, but stopped short of calling for Trump’s removal.

“I think it is vital that he be impeached,” she said.

“Whether removed this close to an election, I don’t know. But I think the conduct is impeachable.”

Asked if she would vote for Trump in 2020, as she did in 2016, she said: “It depends who the Democrats put up.”

Need to catch up with all things impeachment-related ahead of the vote?

We’ve made it easy for you.

SIMPLE GUIDE: If you want a basic take on what's going on, this one's for you

GO DEEPER: Here's a 100, 300 and 800-word summary of the story

WHAT'S IMPEACHMENT? It's a political process to remove a president - video guide

THE VIEW FROM TRUMP COUNTRY: Hear from residents in West Virginia

WHO'S WHO: A guide to the key players in US and Ukraine

A BUMPY RIDE: A newly elected Democrat sells impeachment to constituents

CASE FOR & AGAINST: What legal scholars are saying

Image source, Tom Williams/CQ Roll Call

The Democrats are in a good position - they can lose 18 votes and still reach the 216 votes to impeach the president. But some are expected to break with the party.

So far, only Congressman Jeff Van Drew of New Jersey has opposed impeachment amid criticism from his colleagues, and it is speculated that he will soon join the Republican party instead.

Minnesota’s Collin Peterson is also expected to vote against his fellow Democrats.

But the majority of the other Democrats from districts Trump won in 2016 are still backing impeachment.

Michigan Congresswoman Elissa Slotkin on Monday announced she would vote to impeach the president despite discontent from some of her constituents. Michigan is also considered a state that could swing from Democrat to Republican.

The BBC’s Lauren Turner was with Ms Slotkin when she faced her voters

Anthony Zurcher
BBC North America reporter

Donald Trump had some words for House Speaker Nancy Pelosi and the rest of the congressional Democrats on the eve of his impeachment.

Over six pages, on White House letterhead, the president piled adjectives like cords of wood, fired rhetorical fusillades in all directions and invoked the judgement of the American people, the nation's founding fathers and history itself.

Read Anthony's analysis of the key lines from the president's lengthy rebuke here.

Members of the House are now debating the impeachment rules. These rules will structure how the debate on the articles of impeachment will go forward, setting the parameters for the next six hours.

They'll vote on Article I, abuse of power, before moving on to Article II, obstruction of Congress.

If Republicans delay - which they are expected to - the debate could last up to eight hours.

The president has been calling the impeachment inquiry a sham and a witch hunt from day one.

His re-election campaign - and campaigns by other Republicans running in 2020 - have adopted the same tone.

Many Republican ads have repeated the claim that Democrats have wanted to impeach Trump and overrule the will of the people since the 2016 election.

They say Democrats have succumbed to "crazy" left-wing ideas instead of productive politics and continue to criticise the inquiry as pointless, rushed and partisan.

Republicans have also been insisting this will only hurt Democrats in the 2020 election, specifically targeting vulnerable Democrats in districts and states Trump won in 2016.

But it's worth noting most Democrats did not back impeachment until fairly recently.

And while the 2020 Democratic candidates have backed impeachment and continued to call Trump morally unfit to lead the nation, many campaign ads have opted to draw the focus away from the political circus and onto policies.

Across the country, Democratic campaigns are highlighting issues around the economy and healthcare over impeachment.

It's the same case on the campaign trail, US media report, with candidates veering away from impeachment and voters at town halls doing the same, opting to grill the contenders on issues that affect their day-to-day lives instead.

Image source, Google

It's a big day up on Capitol Hill and Americans are taking notice.

In the past 24 hours, Google searches in the US for "what is impeachment" have overtaken another timely query: "days until Christmas".

And if you are one of those people asking that question, the video below will help.

Or you can read our piece that has all you need to know

So that's what the US Constitution says, but what about the people entrusted to enforce it?

Earlier this month, the president suggested in a tweet that the Supreme Court could step in and stop impeachment.

In an interview with the BBC's Razia Iqbal on Monday, Supreme Court Justice Ruth Bader Ginsburg responded to this.

"The president is not a lawyer, he's not law trained," the 86-year-old justice said.

Read the story in full

And more on the US Supreme Court and why Ginsburg is such an important figure:

• The US Supreme Court: Who are the justices?
• Why half of America panics when this woman falls ill

Image source, Getty Images

Not much.

America’s founding document is rather vague when it comes to the nuts and bolts of impeaching a president.

Article One of the Constitution simply states that the House "shall have the sole power of impeachment", acting as grand jury and levying charges.

The Senate, meanwhile, has "the sole power to try all impeachments" and convict a president of any "treason, bribery or other high crimes and misdemeanours".

As Jonathan Turley, a professor of constitutional law at George Washington University, told the BBC: "The Constitution does not require anything other than a majority vote of the House to impeach a president."

The US Supreme Court has held up the broad powers promised to Congress in the constitution.

In a 1993 case over the Senate's impeachment rights, the justices unanimously ruled that the constitution's use of "the word 'sole' is of considerable significance" and meant the Supreme Court could not intervene - particularly as impeachment is an important check on the judiciary branch by the legislative.

Indeed, the court noted, the word "sole" appears only twice in the constitution - when granting Congress the right to impeach.

From the drama inside Congress on the House floor to the fervour outside.

The possible impeachment of Donald Trump isn't just interesting to lawmakers - pro-impeachment protests have been taking place all over the country in the last 24 hours.

There have also been some supporting the president.

Image source, OLIVIER DOULIERY/AFP via Getty Images

Image source, Erik McGregor/LightRocket via Getty Images

Image source, PHILIP PACHECO/AFP via Getty Images

Image source, ROBYN BECK/AFP via Getty Images

Image source, CBS

Republicans brought a motion to the House floor to adjourn the proceedings completely but it failed.

Democrats - who outnumber Republicans in the House of Representatives - voted no to quash it.

But the Republican motion worked as a delay tactic - the recorded vote lasted about 20 minutes.

Earlier we heard from some duck-owning Democrats in rural Pennsylvania who support the president and hate impeachment.

While in this key swing state, BBC Newsnight's David Grossman also spoke to a Democratic congresswoman who backed the charges - and now has to defend them.

Madeleine Dean won a district north of Philadelphia in the 2018 mid-term elections when Democrats saw a surge of support in the suburbs.

What will impeachment mean for her political prospects in 2020, and that of her party?

She says she is only interested in defending the US Constitution: "I feel an obligation, try not to look at the danger or the downside. I just have a job to do."

Watch David's story.