Four years to the day after a mob of Donald Trump’s supporters violently besieged the US Capitol, Congress will gather to formally certify his re-election.

With Trump’s victory secured, the biggest obstacle currently facing lawmakers this year is a major snowstorm. Vice-President Kamala Harris, who Trump defeated in the 2024 election, will preside over the event as required by the US Constitution.

But the shadow of 6 January 2021 lingers over Monday’s proceedings, despite Trump and his allies’ campaign to recast the attack as a “day of love.”

Heavy security is in place in Washington DC, and the current president, Joe Biden, has vowed there will be no repeat of the violence four years ago, which led to several deaths and millions of dollars’ worth of damage.

The certification, scheduled for 13:00 on Monday, is normally a symbol of America’s commitment to the peaceful transfer of power despite partisan disagreements.

But this time, it has become an emblem of Trump’s extraordinary political comeback and his complete takeover of the Republican Party.

Trump celebrated the moment on Truth Social, writing: “Congress certifies our great election victory today – a big moment in history.”

House Speaker Mike Johnson has vowed to go ahead with the certification in spite of the weather, telling Fox News: “Whether we’re in a blizzard or not, we’re going to be in that chamber making sure this is done.”

Harris, meanwhile, has vowed to “perform my constitutional duty as Vice President to certify the results of the 2024 election.”

“This duty is a sacred obligation – one I will uphold guided by love of country, loyalty to our Constitution, and unwavering faith in the American people,” she said in a video statement.

Ordinarily there would be little need for a vice-president to declare such an intention. The US Constitution requires the certification of a presidential election on 6 January, and for the vice-president to oversee the vote.

But last time the US Congress gathered to certify the election of a US president, the vote was delayed for several hours because rioters, animated by a false belief that the 2020 election was stolen from Trump, smashed through windows, beat their way through lines of police officers, crashed the US House chamber, and ransacked the office of then-Speaker Nancy Pelosi.

In a speech in Washington DC that day, before the violence broke out, Trump told a crowd to “fight like hell” but also asked them to “peacefully” make their voices heard.

Lawmakers, including Republicans, were forced to cower in the basement and Capitol staffers hid wherever they could find shelter. Trump’s vice-president at the time, Mike Pence, was whisked into hiding as rioters erected a gallows on the Capitol grounds and called for his hanging because he refused to inaccurately certify the results in Trump’s favour.

In the aftermath, Capitol Hill custodial workers worked furiously to clean up shattered windows, trashed corridors, and even human waste, an experience one described as “degrading.” Congressional staffers spent the next several months reckoning with the trauma of the attack.

The riot caused nearly $3m (£2.4m) in damage, injured over 100 police officers, and shocked America’s political system.

In the immediate wake of the attack, which millions of Americans watched unfold on television and social media, there was little debate over who deserved blame.

The US House of Representatives impeached Trump on charges he incited the riot, but the US Senate did not achieve the two-thirds vote required to convict him. Then-Senate Majority Leader Mitch McConnell, a Republican, explicitly blamed Trump, saying rioters “did this because they had been fed wild falsehoods by the most powerful man on Earth – because he was angry he’d lost an election”.

Trump himself faced federal charges for allegations that he attempted to subvert the 2020 election, to which he pleaded not guilty. But the Department of Justice (DoJ) was forced to drop the case once he was elected, due to protocols that prevented the prosecution of a sitting president.

As Trump sought a return to power, he and his allies have worked to dramatically change the narrative around the riot and its cause.

Trump said there was “nothing done wrong at all,” at an October 2024 presidential campaign forum.

He has called the people convicted by the DoJ “hostages” and “political prisoners.” And his new vice-president, JD Vance, refused to acknowledge in a presidential debate that Trump lost the 2020 election.

Americans now have starkly divided perspectives on the day. A January 2024 Washington Post/University of Maryland poll suggested that a quarter of Americans believed a conspiracy theory that the FBI instigated the attack. While a majority of Americans believed 6 January 2021 was an attack on democracy, only 18% of Republicans thought so, the poll indicated.

Trump captured all seven of the country’s swing states during the 5 November presidential vote, giving him a resounding victory in the electoral college, the mechanism that decides who takes the presidency.

It will be Harris’s job on Monday to read out the number of electoral college votes won by each candidate.

Trump’s second term will begin after he is inaugurated on 20 January. For the first time since 2017, the president’s party will also enjoy majorities in both chambers of Congress, albeit slender ones.

It represents a stunning political comeback from his electoral defeat in 2020, and a criminal conviction in 2024 – a first for a current or former US president.

Trump’s pledges after returning to office include pardoning people convicted of offences over the attack. He says many of them are “wrongfully imprisoned”, though has acknowledged that “a couple of them, probably they got out of control”.

Conversely, Biden has called on Americans never to forget what happened.

“We must remember the wisdom of the adage that any nation that forgets its past is doomed to repeat it,” Biden wrote in the Washington Post over the weekend.

For Trump’s Republican Party, the new Senate Majority Leader John Thune has signalled a desire to move on, telling the BBC’s US partner CBS News: “You can’t be looking in the rearview mirror.”