Yvette Cooper will launch a direct assault on Jeremy Corbyn’s politics, saying that “they aren’t radical, and they aren’t credible” in a speech in Manchester later today.

The shadow home secretary, whose campaign has been criticized for its overly cautious approach, will surprise her detractors with a frank speech that, she will concede, “may lose me votes”.

Saying that “there’s a battle on for the soul of our party”, she will challenge to those supporting Corbyn’s bid for the leadership, she will ask them to “tell me what you think is more radical: spending billions of pounds we haven’t got switching control of some power stations from a group of white middle aged men in an energy company to a group of white middle aged men in Whitehall as Jeremy wants, or extending Sure Start [the early years programme Cooper helped set up in government]; giving mothers the power and confidence to transform their own lives and transform their children’s lives for years to come.”

She will describe George Osborne’s plans as “way beyond what is needed to bring the deficit down”, calling it “punishment not prudence”, and “deliberate dismantling of the state”.

Osborne’s plan, she will argue, highlights the need for a real alternative – but “it also has to be credible: and Jeremy’s isn’t”. Cooper, the only trained economist in the race, will describe his plan for further quantitative easing as “really bad economics”, saying “no good Keynesian would ever call for it” .

She will challenge “this idea that power doesn’t matter so long as our principles remain intact”, saying “I dare you to tell that to the woman in tears because she can’t afford her bedroom tax arrears, tell that to the working parents on tax credits about to lose thousands of pounds who cant afford new school shoes for the autumn term, tell that to all those people who are being hit by Tory government”.

She will conclude “it’s not enough to be angry at the world, we’re the Labour Party – we have a responsibility to change the world, or what’s the point of us at all?” With polls still showing large number of undecideds, her campaign will hope the intervention is enough to shift things in her favour.

On the battle facing Labour:

If the Times and You Gov are right – and it’s a big if – there’s a battle on for the soul of our party.

It’s not about personalities. It’s about the future of our country, and I won’t duck that fight, because there is too much at stake. I’m here because I can’t walk away from this party, can’t walk away from the people Labour should be standing for.”

On why people are supporting Corbyn:

When times are tough, and the old answers, and the old parties don’t seem to be working, people cast around for something else. Something different. Something subversive. That’s part of why many people want to buy into what Jeremy is offering. I’m not going to dismiss the values and the intentions of Jeremy and those who are supporting him – I don’t think that’s right. But nor am I going to pretend I agree with them on the answers. Because the truth is that Jeremy is offering old solutions to old problems, not new answers to the problems of today

On why she’s giving the speech:

Saying this rather than pretending I agree with the most popular candidate in the race may lose me votes. But it needs to be said. Our party, the values we stand for, and the country we want to fight for are too important not to be honest about what is at stake. And I want to show today that there is an alternative that is both radical and credible, true to our values, but serious enough to win And we have to fight for it before it is too late.

On Nato membership:

And at a time when we are dealing with a global climate change threat, when international borders have ebbed, when extremism doesn’t recognise nations, and when we need to work together more than ever, is it really radical to quit Nato, to prevaricate over membership of the EU, or trash our reputation as an internationalist party? I say no. We should stay in the EU, stay in the European Court of Human Rights, stay in Nato – sorry, Jeremy, internationalism is a core Labour principle and I will always fight for it.

On “principle over power”: