Steve King, a firebrand Republican from Iowa, has called the impeachment inquiry an effort by Democrats to shield former Vice-President Joe Biden's work in Ukraine.

"This is the largest and most massive cover-up of such a list of crimes against our country," he says.

Biden, who served under President Obama, was one of many US and EU officials who was calling for the ouster of a Ukrainian prosecutor who was widely viewed as corrupt.

Republicans argue that Biden's efforts were made to protect his son, who was working for a Ukrainian energy firm at the time. He strongly denies this.

"The reality is it was Biden that was doing the extortion... in order to protect his own son," says King.

"And it was Donald Trump that was following the law" which required him to ensure there was no corruption in Ukraine before releasing US military aid.

Our Ukraine correspondent, Jonah Fisher, recently took a closer look at the allegations.

Louisiana Republican Clay Higgins is not exactly toning down his arguments today.

"I have descended into the belly of the beast," he declares from the podium, slamming the impeachment hearing.

"I have witnessed a terror within. And I was committed to oppose the insidious forces which threaten our republic."

It's similar to language the "Cajun John Wayne" used when declaring a congressional run in 2016, referring to his campaign as "descending into the belly of the beast."

"I'm descending as a soldier by the will of the people, descending as a warrior armed with the Constitution," declared the former sheriff, who carried a chrome-pistol at his side while he spoke out then against "career politicians who no longer serve we, the people".

Before joining Congress, Higgins became famous for his YouTube channel, in which he insulted criminals and called for them to turn themselves in.

Watch our video of his criminal call outs from 2015:

'Cajun John Wayne' calls out criminals

Image source, Getty Images

Today's debate has been scheduled for six hours.

We'll forgive you if you prefer to spend that time doing something else.

Here are some suggestions:

• Watch the Wolf of Wall Street - twice

• Run a marathon (and a half)
• Cook a 24lb turkey
• Finish season one and two of the BBC's Fleabag
• Watch Meistersinger - the longest show in the history of New York's Metropolitan Opera House
• Read Harper Lee's To Kill a Mockingbird
• Grab a car and get out of the country, almost - Canada is about a seven-hour drive away from Washington, where impeachment proceedings are still rolling along

Image source, Getty Images

Today's debate and vote is open to members of the public - as have the impeachment inquiry hearings that have been going on in recent weeks.

So who actually goes along to watch them?

As Politico found out, activists, retired people and parents who homeschool their children, giving them a live lesson in civics.

Here's their take on an earlier hearing in the process, external.

Let's take a wider look at what's going on.

Here you can catch up with all things impeachment-related.

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

Vice-president Mike Pence - who the constitution says will replace Trump in the unlikely event that the Senate votes to convict him in January - has just weighed in on today's House vote.

Pence, a former Indiana Congressman, said at a "Workers for Trump" event in Saginaw, Michigan that itwas a "sham investigation".

"Do-nothing Democrats are actually going to vote on a partisan impeachment seeking to overturn the will of the American people.

Pence called on Democrats across the aisle to "put partisan politics aside, end this partisan impeachment and get back to work on the issues that matter most to the American people".

Image source, PhotoQuest/Getty Images

Donald Trump is the third president to face a House vote on impeachment.

The first - Democrat Andrew Johnson - was impeached in 1868, after a contentious battle with Republican lawmakers.

The final straw for Johnson was his firing of cabinet member Edwin Stanton - a political rival of the president.

If you think the Ukraine story has provided a lot of political drama, it's in keeping with an impeachment tradition - Stanton responded to his dismissal by locking himself in his office and refusing to leave.

After impeachment, Johnson was saved by just one vote in the Senate and was able to serve out the rest of his term.

This from earlier today...

Trump has issued his own battle cry hours before heading to Battle Creek, Michigan, for a "Merry Christmas" rally tonight at 19:00 local time (midnight UK time).

Perhaps indicating his mood, the president tweeted in capitals, denouncing the "atrocious lies by the radical left".

The White House spokeswoman has pledged that he plans to address today's events at the rally.

According to the town newspaper, the Battle Creek Enquirer, it is the 30th largest city in the state, and the 710th largest in the nation.

So why is he going to this small city, population 52,347?

Trump won a narrow victory in Michigan - a state that Democratic presidential candidates had carried since 1988 - and is campaigning to keep the swing state in 2020.

The name of the city might also have a certain appeal to the president, as Washington engages in a partisan street fight.

Elsewhere in the state of Michigan, voters confronted Democratic congresswoman Elissa Slotkin at a town hall meeting - hours after she came out in favour of impeachment.

Here's what happened at the event.

And this, below, gives a flavour of what people can expect at Trump's rally.

There have been anti-Trump protests going on ahead of today's debate and vote.

But there are also those who are flying the flag for President Donald Trump in the capital city, as these images show.

Image source, Getty Images

Image source, Getty Images

Image source, Getty Images

Peter Sartorio, a barber in small town Pennsylvania, says his customers are not talking to him about impeachment when they sit down for a haircut.

"The trouble with impeachment is that it's taken too long and people lose interest," he says.

"I think people don't care too much about it. They're more worried about their healthcare, more worried about jobs and everything else that goes with the economy."

Peter was talking to BBC Newsnight's David Grossman who has been testing the mood of voters in Luzerne County, a place that voted for Barack Obama in 2008 and 2012 but turned to Donald Trump in 2016.

Watch his full film to meet a Democratic couple who love Trump and hate impeachment - and also a member of Congress who says she has an obligation to defend the Constitution whatever the political risk.

New York Democrat Jerrold Nadler, the House Judiciary Committee chairman, says: "Congress cannot wait until the next election to address this conduct," due to the threat Trump poses to the upcoming election

"When President Trump conditioned military aid on personal favours, he harmed America's national security," he says, referring to Trump's call to the Ukrainian president.

"We must act without delay," Nadler continues. "His actions warrant impeachment and demand removal from office."

Let's take a step back for a minute and recap on what Trump is accused of.

Democrats allege the US president abused his power in trying to extract political favours from Ukraine by withholding military aid and a White House meeting.

The most serious allegation of pressure on this US ally is that he asked for an investigation into his political rival Joe Biden, thereby enlisting foreign help to win the 2020 election.

The second article of impeachment is obstruction of Congress, because White House aides were ordered not to testify in the inquiry and requests for documents were refused.

Read the video below for a fuller picture of the road that led to this point.

There are two key assertions being made right now by Republicans on the floor - defending the president and claiming the entire process is unfair.

The president has also done both, describing his dealings with Ukraine as "perfect".

Today is a historic day in the US - the events in Washington are the subject of wall-to-wall press coverage across the country.

But this isn't a first.

In 1975, Richard Nixon resigned in the face of near-certain impeachment and removal from office. Just over two decades later, Bill Clinton refused calls for his resignation, and hung on to power through his own impeachment in 1998.

Here's a look at how two of the country's top newspapers covered those past presidential scandals.

Image source, New York Times

Image source, Washington Post/Getty Images

Image source, New York Times

Image source, Getty Images

Image source, Washington Post

Image source, New York Times

House lawmakers are still having their back and forth, with Speaker Pelosi still at the podium.

Let's cross to the upper chamber where her equivalent in the Senate spoke earlier.

Republican leader Mitch McConnell said: "The country is waiting to see whether these House Democrats will give into the temptation that every other House in modern history has managed to resist and misuse the solemn process of impeachment to blow off partisan steam."

Senate Democrats, meanwhile, brought up McConnell's 1998 remarks during Clinton's impeachment to criticise him on Twitter.

The top Democrat in the House, is now speaking on the floor.

Ms Pelosi begins by quoting the US pledge of allegiance.

Nobody comes to Congress to impeach a president, she says, pledging "to support and the defend the Constitution from enemies both foreign of domestic" - quoting the oath taken by congressional lawmakers.

"Now our founders' vision of a our republic is under threat from the White House," she says, calling it an "established fact that Trump violated the constitution".

"As speaker of the house I solemnly, and sadly, open the debate on the impeachment of the impeachment of the president of the United States," she says from the podium.

Trump's "reckless actions make impeachment necessary," she continues, calling Trump an urgent threat to US elections.

A simple majority is, as it says on the tin, a majority in its literal terms - 51%. A supermajority in Congress requires two thirds of the vote. Here’s what processes require what support.

• To impeach the president: Simple majority
• To determine the format of the trial, including witnesses: Simple majority
• To dismiss the charges after questioning: Simple majority
• To convict the president and oust him: Supermajority
• To bar the president from running for office again after a conviction: Simple majority

Democrats control the House 233-197 while Republicans hold the 100-seat senate 53-47.

Image source, Win McNamee/Getty Images

Now that House members have concluded debate on the rules for impeachment, debate on impeachment itself has begun.

They're here for the long haul - debate is expected to last at least six hours and could go up to eight if Republicans try to delay.

Two Democrats, Jeff Van Drew and Collin Peterson have defected already, voting with Republicans to end today's proceedings. We'll stay tuned to see how these lawmakers vote throughout the day.

Once debate has concluded, they will vote on the two articles of impeachment - abuse of power and obstruction of Congress - individually.

Image source, David Hume Kennerly/Getty Images

In the case of President Bill Clinton, the answer is a resounding yes.

Clinton resisted calls for his resignation over his affair with a White House intern, and his grit paid off.

He was impeached in the House in 1998 for lying about the scandal, before being acquitted in the Senate, as both chambers largely voted along party lines.

When he left office after his second term, Clinton boasted a 65% approval rating - the highest of any of his predecessors in half a century.

But, but, but...

His party lost the White House for two terms.