Seema Malhotra, the shadow chief secretary to the Treasury, could hardly be more different from her boss and constituency neighbour in west London, John McDonnell.

While the shadow chancellor – and Jeremy Corbyn’s righthand man – cites Marx as a major intellectual influence, Malhotra, a rising Labour star, is a former management consultant who grew up above a shop in Hounslow, the daughter of Indian immigrants, and speaks in glowing terms of the contribution business can make to society. Halfway through an interview dissecting the Tories’ economic policies, a division bell rings, and she swaps her heels for trainers and bolts off to vote.

Anything but a paid-up Corbynista, the MP for Feltham and Heston was “very active” in Yvette Cooper’s campaign for the Labour leadership and she shares some of Cooper’s caution about elements of Corbynomics, warning that so-called People’s QE “wouldn’t be the answer” if used at the wrong time in the economic cycle, for example. But she accepted a senior shadow role, because – notwithstanding the chaos at the top of her party – she believes Labour now has the best chance since 2010 of making its economic argument heard.

In this Wednesday’s autumn statement, the chancellor will have to reverse, or at least soften, the deep cuts to tax credits that have enraged his own backbench MPs and were overturned in the Lords. Malhotra believes what she calls the “tax credits fiasco” will come to be seen as a turning point for Osborne: the point at which he lost the trust of the public.

“Some would say: ‘did you lie to the British people at the time of the general election?’ Every wing of the Tory party was out saying ‘we won’t be cutting tax credits’, so for those over three million families who are claiming tax credits – 7,500 of them in my own constituency – they will be saying that this is a government that didn’t tell us the truth.”

With Osborne reeling from these self-inflicted wounds, Malhotra believes Labour must seize the moment to make its case – particularly after McDonnell executed his own U-turn, deciding not to back the Tories’ “fiscal charter”, which commits the government to achieving a surplus on the public finances.

McDonnell initially believed that not supporting the charter would be an error that would open Labour to the charge of being reckless with taxpayers’ cash. But Malhotra was among those who persuaded him to change his mind, and Labour is now increasingly hopeful that the charter will become a millstone around Osborne’s neck as the cost of austerity becomes clearer.

With the health secretary locked into a pay dispute with junior doctors, and the prime minister found out complaining to his own council leader about the impact of local government spending cuts, Malhotra says: “More and more people are getting caught into arguments about choices the Tories are making and why.

“You have very accomplished backbenchers like Heidi Allen and Steve McPartland actually challenging their own frontbench: I think that has had a big impact on trust in Osborne.”

Facebook Twitter Pinterest John McDonnell, the shadow chancellor. Photograph: Steve Parkins/Demotix/Corbis

She and McDonnell plan to press home the idea that many of this latest round of cuts will amount to a false economy – saving pennies in the short term but undermining the foundations of growth, and thus ultimately self-defeating. “There’s something that doesn’t quite add up about what George Osborne is doing. He is essentially saying that you can cut your way to prosperity, and he’s doing that without looking at the deep impact of what he’s about to do.”

While the thrust of this argument echoes the approach of McDonnell’s predecessor-but-one Ed Balls, who blamed austerity for the flatlining of the economy after 2010, today’s shadow Treasury team believes the notion of false economies chimes more closely with families’ everyday experience.

More importantly, though, Malhotra argues that Osborne can now be held to account on his own record. “We lost time, particularly at the start of the last parliament, in making a stronger argument for Labour and our economic policy. But what’s different now is that we are five years on, and we have a chancellor that can’t make excuses any more.”

And while an internecine war is raging in her party across a range of fronts, including defence, Malhotra says McDonnell is quietly rebuilding economic policy from the ground up, helped by his much-vaunted advisory council of economic radicals, including Thomas Piketty and Mariana Mazzucato. “I feel confidence about the foundations we’re setting, about the long-term thinking, about the team that we’re building, and about the wider engagement we’re bringing in,” she says.

“We want to have a different conversation about how you generate growth that can be spread more fairly, that can support small and large business, that can help with the pathway of turning a good local idea into something that becomes global.”

That’s a somewhat different tone to that struck by Corbyn in his leadership campaign when he talked of slashing “corporate welfare”, but Malhotra says her sympathy for businesses springs from a childhood above the family shop.

As the time allotted for the interview ends, she closes her book of carefully prepared notes and jettisons the calm tones of the well-rehearsed soundbite. “We sold school uniforms, clothes, accessories, thread, buttons; it was a proper community hub,” she says. “Growing up in a small business, you see that small businesses are the lifeblood of local economies. Small business owners are combining work with family, and for me it was the first introduction to public service – serving customers that came into the shop.” She had no overseas holidays until she saved up to go to India aged 18, and her parents sometimes struggled to make ends meet. “ I know that it was not uncommon for there to be an overdraft at the end of the month.”

Those days are far behind – headlines during the election campaign focused on the Chelsea “mansion” she shares with her wealthy husband. But it’s clear that the long shadow of that humble childhood has allowed her to make common cause with the Corbynistas. “When you have choices about whether and where you support families – families that are struggling for whatever reason, because of unemployment, because of low income, because there might be health conditions in the family that can turn a fortune around overnight – you have to have a politics that’s going to be on the side of people in tough times.”

For now, Malhotra is the likable face of the uneasy compromise Labour’s political centrists have struck with their firebrand leader. Only time will tell how long it can hold.