It was revealed this week that the Tories have seen a dramatic fall in their popularity among ethnic-minority voters. A private Downing Street poll showed support plummeting to its lowest level since Iain Duncan Smith was party leader.

For more than 20 years we at Operation Black Vote (OBV) have campaigned for the parties to take minority voters more seriously, warning that the size of Britain’s black electorate shows it has the power to swing a general election. After these figures, and their likely impact on the general election result, it seems we’ve finally been proven right.

Just take a glance at some of the parliamentary seats that changed hands this year, and we can begin to see that movement within a relatively small number of seats can mean a huge difference: Bedford, Bristol North-West, Derby North, Reading East, and Peterborough; in London, Battersea, Enfield Southgate, Croydon Central, and Kensington. All have significant black or Asian populations.

It’s no surprise to me, then that No 10 has analysed the data, scrutinised the feedback and concluded that, if the Tories had won over the “black vote” in key marginal seats, they would now be enjoying a parliamentary majority.

Facebook Twitter Pinterest ‘In one visit to a Sikh temple, David Cameron wore a traditional orange patka.’ Photograph: Peter Macdiarmid/Getty Images

So what went so badly wrong for the Conservatives? For some years political leaders have recognised that with a diverse society you need to have an appeal to the whole population on the one hand, but also to specific groups, such as pensioners, young people, women, and black and minority communities. During the 2015 general election campaign, the Tory leader David Cameron went to the annual Festival of Life – and spoke to more than 40,000 black churchgoers, bringing an inclusive message: “I believe in aspiration ... I look out to this crowd and I see someone who will take my role and become prime minister for this great country.”

He took the same message to other faith groups too – in one visit to a Sikh temple he wore a traditional orange patka – extolling their business and family virtues. Even without much substance, that pulpit narrative helped get him over the line to a majority government. Fast forward two years, and in a snap election Theresa May struggled to attract minority voters. On her first day as prime minister she had acknowledged: “If you’re black, you’re treated more harshly by the criminal justice system than if you’re white.” But her genuine desire to confront persistent race inequality was completely lost in the relentless mantra of “strong and stable”, “strengthen my hand” or “coalition of chaos” that marked her election campaign.

By contrast, Labour’s Jeremy Corbyn had a strong message of social justice, which plays strongly among communities that face discrimination on a daily basis and bear the brunt of austerity. As a perennial outsider, Corbyn’s belief in this cause has never wavered, but this time round his masterstroke was something that mainstream politicians rarely do: that is, look beyond the 60% of people who traditionally vote.

We at OBV have always argued that this untapped constituency, which is dominated by ethnic minority, working class and young voters, could render any seat winnable. Any seat. Corbyn sought to make friends and listen to a sector of society that was never on the radar of major political parties; this included black grime artists, who had hundreds of thousands of social media followers. In addition, Corbyn has a number of prominent black and Asian faces on his front bench. Thousands of young black first-time voters felt empowered, registered to vote, and voted in record numbers.

So what now for May and Corbyn?

May knows that the Labour leader stole a march on her, but she is responding. Last month she appointed the black activist Nero Ughwujabo as a special adviser who will work closely with her on race issues.

In fact, though it was barely mentioned at this election, May has a good track record on race. She confronted the police over black deaths in custody and high levels of stop and search; and I know she’s aware that, if unchecked, race inequality will widen. That’s why she launched the historic race disparity audit; and depending how she follows it up she could have a strong platform to demonstrate a commitment to tackling burning racial injustices.

But so far she’s been held back: first and foremost by her own hardline Brexiteers, who grab the headlines with a narrative that is widely perceived as anti-foreigner. Many minorities have a genuine fear that a hard Brexit will mean their equal rights protections are torn up in a “bonfire of the regulations”.

This budget will make things even worse for women and the disadvantaged | Dawn Butler Read more

For Corbyn, complacency will be the biggest challenge. For decades Labour has taken the black vote for granted, offering lip service to equality, but then continuing with business as usual. Cameron saw this and seized his opportunity in 2010 and 2015. And some early signs from Labour are troubling, such as the case of Bilal Mahmood. He twice fought as a candidate for Labour against Iain Duncan Smith in the safe seat of Chingford and Woodford Green, and made a big dent in the Tory majority. But the central party machinery then ousted him and introduced an all-women shortlist: these have historically done little for black and Asian candidates, and of course exclude Black and Asian men, who are also under-represented.

In politics, as the past 18 months have shown, there are few certainties; but one of them is the growing influence of the ethnic-minority electorate that will continue to demand greater social and racial justice. Politicians ignore this at their peril. If there’s one thing we’ve learned this year, it’s that black votes matter.

• Simon Woolley is the director of Operation Black Vote