If Boris Johnson gains a majority tomorrow, we are at risk of entering a political dark age dominated by extreme rightwing populism.

We will face a Johnson government – with its establishment allies owning most of our print newspapers, with a proven ability to bully and dominate broadcast media, and a capacity to buy up dark ads across social media – that is able to manipulate our politics on a scale we have never seen in this country.

They will claim to be ending austerity – the cuts that have resulted in more than 700 of our fellow citizens dying homeless on our streets last year, and left sick children forced to sleep on floors in our hospitals – but that austerity is still “baked in” to their spending plans, in the words of the Institute for Fiscal Studies.

Spending plans that will rapidly deteriorate as Johnson’s hard Brexit has its impact – or the catastrophic chaos of “no deal” is put back on the agenda by the European Research Group.

Scared of Corbyn? As a black Jewish woman I’m terrified of Johnson | Nadine Batchelor-Hunt Read more

If anyone is deluded enough to believe that Johnson is anywhere near being a “one-nation” Conservative, they need to peruse just a small sample of his writings and speeches over the years. On top of his notorious racist, homophobic, antisemitic and misogynist comments, he obviously holds in contempt anyone who is not a member of his Eton and Bullingdon Club set. To Johnson, we are all just suckers – easily manipulated by his fabricated boyish japes and his deliberately tousled blond hair.

Last week his father betrayed the Johnsonian attitude to working people when he doubted they were literate enough to know how to spell Pinocchio. This is the attitude that has created obscene inequality in our society – and justifies that inequality as some sort of natural order.

As inequality grows and the hopes of a generation fade away, it will be easy for the populist right under Johnson to divide and rule by mobilising people against one another, shifting blame from the powerful – just as he did in the Brexit campaign.

Play Video Boris Johnson is least trusted politician, says John McDonnell – video

But where the risk of a Johnson majority moves from obnoxious to downright dangerous is on the existential threat we face from the climate crisis.

Science dictates that the next decade is critical if we are to stand any chance of tackling climate change. Despite the electoral stunt of pausing fracking for the election period, we know that Johnson and the Tories are backed by the fossil fuel companies and their financiers. Johnson’s manifesto not only fails to address climate change – its plans for major road building and aviation expansion will take us backwards.

Nobody interested in creating a more equal, more socially just, fair, prosperous and sustainable society has any longer the excuse that there is no party to vote for.

Labour’s manifesto aims to create a society that is radically fairer and radically more equal – based upon a prosperous economy whose prosperity is shared by all, and achieved by protecting, repairing and enhancing our environment.

We have produced this ambitious manifesto because we recognise that we now face twin emergencies. A social emergency caused by the brutality of nearly a decade of unnecessary austerity that has undermined our public services and the quality of life and life chances of our people. And a climate emergency that threatens our very existence.

With a fair taxation system – reversing some of the £100bn given away to the rich and corporations by the Tories – and investments for the long-term in our economy, we can end austerity, rebuild our public services and stop the privatisation of our NHS.

With a large-scale green new deal, including a green industrial revolution, we can step up to the plate in making our contribution to saving our planet.

This is no time for equivocation. No time for “if onlys” – for “I would vote Labour if only it had been solely a remain party”, or “I would vote Labour if only they changed their leader”. No time for fantasies that some new centrist party will miraculously emerge from the debris of a Labour defeat and the unpopularity of a Johnson regime.

When people look back on this week many will ask us all: “What did you do to end that world of rough sleeping and food banks?

“What did you do when they came to sell out the NHS?”

“What did you do when our planet was at risk?”

I want you to be able to tell them: “I helped house the homeless, end the poverty, halted the sell-off of the NHS and made my contribution to saving our planet.”

“It started when I voted Labour on 12 December 2019.”

• John McDonnell is the shadow chancellor