The Conservative election campaign has been built on a platform of lies – but it crashed to a new low with the grotesque manipulation of the London Bridge terrorist attack. For Boris Johnson, the murder of two remarkable young people who were committed to helping others is merely a campaign prop.

Where others reacted with horror and grief, Johnson and his team saw an opportunity to electioneer. That they used this as a chance to promote punitive penal policies which one of the victims, 25-year-old Jack Merritt, devoted his short life to fighting against, almost beggars belief. To bolster the case, the Tories again fall back on lies: Johnson claimed parliament blocked plans to prevent automatic early release of prisoners (false, it was passed by 16 votes); that opposition parties were to blame for the terrorist’s release in 2012 (false); that Labour introduced legislation allowing the early release of terrorists (false); that Jeremy Corbyn wants to disband MI5 (false). The lies provide a shield from their own record: the decimation of prison and probation budgets. An ex-prosecutor even claims that Johnson told him that there wasn’t enough money to fund deradicalisation programmes.

Yet how to properly confront such deceit when so much of the media exists to facilitate rather than scrutinise or challenge the Conservative party? There is little if any demarcation between the Tory election apparatus and many British newspapers. They are one unified, integrated, ruthless machine – and should be treated as such. As front pages laud Johnson’s manipulation of a horrific incident, the pleas of Merritt’s father, David, go unanswered: “Don’t use my son’s death, and his and his colleague’s photos – to promote your vile propaganda. Jack stood against everything you stand for – hatred, division, ignorance.”

Then there is the Andrew Neil debacle. All leading politicians should face a level playing field during an election campaign: given the partisan loyalties of most newspapers, this is impossible, but it is the least we should expect from our public broadcaster. And yet the BBC responded to Friday’s atrocity by performing a U-turn on their decision to block Johnson’s interview with Andrew Marr until the prime minster agreed to subject himself to Neil’s rottweiler treatment. The BBC instead allowed Johnson to use the atrocity to grandstand and repeat untruths which were only later challenged by a BBC Twitter account. The BBC has allowed terrorism to interfere with British democracy. As Phil Harding, the BBC’s former controller of editorial policy and editor of the Today programme, put it, the BBC decision allowed Johnson “to appear ‘prime ministerial’” and made the attacks an “election issue”.

The British right has long fantasised about privatising and gutting the BBC while much of the left has long criticised the corporation’s editorial decision-making, but now even the centre is turning against it. This election campaign has been the BBC’s nadir: from editing out Johnson being ridiculed by a Question Time audience – simply a mistake, we are told – to supinely publishing video of him eating scones in a fashion that would embarrass North Korea’s official news agency. The counterweight to our rightwing press is fast being eroded.

And then creeps in the idea that the problem lies with both sides of the political divide – muddy the waters, ascribe blame equally for dishonesty. It works, of course: speaking to voters on doorsteps across the country, I hear the constant refrain that “all politicians lie”. But it’s too easy an accusation to level, and one that is bolstered by casual asides from senior BBC presenters such as Emily Maitlis, like the idea that this is “the election of 2 billion trees and 50 thousand nurses – where numbers and accountability became meaningless”. The comparison between Labour’s mass tree-planting commitment – an achievable aim that has been accomplished in other countries – and a Tory policy on nurses which we know is bogus, because the figure includes retaining existing nurses, is hugely damaging.

It is easy to conclude that this is merely barking at thunder. It rains on bank holidays, buses come in twos, and the media landscape favours the Conservatives. And it is certainly a deliberate tactic from the Tories to keep the left relentlessly bogged down in outrage at their lies. But if the Tories are allowed to lie unchallenged then our democracy will wither. While Labour should sped the remainder of the campaign focusing on the Tories’ wretched record in government, the threat the party poses to the NHS and promoting its radical domestic agenda, it is beholden on those of us with a platform to challenge the Tories’ systematic deceptions. The aim is not simply to defeat Johnson, but to defend a democracy in mortal danger.

• Owen Jones is a Guardian columnist

• This article was amended on 2 December 2019. The government wants to prevent automatic early release of prisoners, not allow it