In the race to be the new Tory leader, Theresa May has a stonking lead, her pitch being that she is the only grown-up in the room. And when you look, she has a point. Narcissist Boris Johnson and “political psychopath” Michael Gove have both fallen by the wayside. And whilst it’s great to see an all-women shortlist for Britain’s next prime minister, Andrea Leadsom still looks amateurish and is flirting with Ukip.

So what would a Tory government led by May be like? Don’t believe the hype about a safe pair of hands or uniting the country – there are huge risks for Britain ahead that her politics won’t solve. Indeed, division will grow. And she certainly isn’t too tough to defeat when the general election comes.

For 19 years I’ve seen how she works. Both elected in 1997, we sat on our first select committee together. She shadowed me when I was work and pensions secretary. I shadowed her as home secretary.

May believes in justice, but not in social justice; in individual enterprise, but not in uniting communities. Rightly, her Modern Slavery Act promised a crackdown on people smugglers. Wrongly, it left out protection for domestic workers from slavery. Rightly, she criticised poor standards in policing. Wrongly, she destroyed the neighbourhood policing that builds community cohesion and prevents crime. Rightly, she talks about entrepreneurship and getting on in life. Wrongly, she never challenges the deep inequalities and poverty that hold people back.

For six years she has been at the heart of a Tory government that has cut taxes for the richest and cut tax credits for the poorest. So when she suddenly claims she wants people to understand how hard it is for working-class families, she should admit that her government has made it much worse.

I respect her style – it is steady and serious. She is authoritative in parliament – superficial attacks on her bounce off. When the Tory establishment call her “a bloody difficult woman” she rightly wears it as a badge of pride. But the flip side is that she is not fleet of foot when crises build, she digs in her heels (remember the Passport Agency crisis in 2014 when the backlog caused hundreds to miss their holidays, and the Border Force crisis in 2011 when border checks were axed).

And she hides when things go wrong. No interviews, no quotes, nothing to reassure people or to remind people she even exists. It’s helped her survive as home secretary – but if you are prime minister, eventually the buck has to stop.

The current row about the fate of EU citizens settled in the UK sums her up. The home secretary is refusing to say they can stay. Under pressure she’s gone to ground, sending a junior minister to parliament to defend her position instead.

It’s not just technocratic caution. This potential prime minister wants to look tough to the Eurosceptics and she is stubbornly refusing to recognise the anxiety of families across the country settled here for years but now worrying about whether they need to move home, job or their kids’ school. Even more troubling, she is cavalier about the fact that this uncertainty is undermining community cohesion and giving succour to vile extremist “go home” campaigns.

There’s a common theme here. May and the strand of conservatism she represents have never seriously understood or cared about community cohesion and solidarity. And nowhere is that more true than on immigration. On her watch, anxiety about this issue has grown so high it has destroyed our relationship with our closest allies in Europe. She whipped it up over years with rhetoric and repeated failed promises. Then in the referendum campaign itself, when it became the key issue, she disappeared: a dereliction of duty.

She never had a serious strategy to address public concern – to stop the undercutting of pay, to improve public services, train British workers to do the jobs, or change attitudes with an honest debate. Nor was she ever able to build the steady, slow alliances to reform free movement from within the EU, having wasted political capital in a pretend row over the European arrest warrant. Immigration has become toxic on her watch, and divided our nation. Where we urgently need to build a new consensus, this home secretary never had a plan.

To beat May, Labour must be a strong and effective opposition. We must be as sharp and focused as she is, to hold her to account in the Brexit negotiations and offer alternative, progressive plans each time she falters. We must be swift on our feet to chase after her and her ministers each time they run and hide. We need an optimistic vision for Britain with social justice, equality and community cohesion at its heart. And we must be able to unite a divided country, because we know the Tories won’t and can’t.

Be in no doubt about how much is at stake. Because while everyone blames David Cameron’s misjudgment, Boris Johnson’s ego and Michael Gove’s lies for the divided country we now live in, we should not forget how much responsibility lies with the woman who seems likely to be handed the keys to No 10.