Dig beneath the topsoil of the East Midlands, and you will find Labour, in all its layers.

This unassuming region includes some of the most overt of Corbynsceptic MPs, as well as his most vociferous allies. It is home to some of the country’s most deprived post-industrial former pit towns, still reeling from the Thatcherite treatment, and some of its most green and pleasant lands. Some East Midlands voters will die before they vote Tory. Others have gone blue – or purple – with barely a pang.

Some of the most Leave-voting seats anywhere in the UK are in the East Midlands.

Dennis Skinner’s Bolsover – which contains Shirebrook, where Sports Direct HQ sits metaphorically on the site of the old pit – returned a 70.8 per cent Leave vote. The Beast of Bolsover is just one of the big-hitters of the region. In the blue corner sit the Tory party's most outspoken Remainers – Anna Soubry, Nicky Morgan and Ken Clarke, no less. Then there are Labour MPs of all shades of red – Gloria De Piero, Chris Leslie, Vernon Coaker, Lilian Greenwood, Chris Williamson, and Liz Kendall.

There has been plenty of temper and feeling. As Greenwood spoke in favour of Owen Smith at last year’s Nottingham East constituency Labour nomination meeting, a leaflet denouncing her recent resignation from the shadow cabinet was passed by hand around the hall. A year earlier in the same constituency, Williamson – visibly distressed at having lost his seat by 41 votes a few months before – gave a furiously impassioned plea for members to get Jeremy Corbyn into the top spot. (Afer winning the seat back, he declared that Corbynsceptic MPs should be "on bended knee, apologising").

But if the East Midlands is known for its Labour-or-die towns and cities, in June, Mansfield dropped off this list for the first time in 94 years. This was previously unthinkable. As a local member told me after the election: “In towns like this your dad voted Labour, his dad voted Labour and his dad voted Labour. They’d be turning in their graves now.”

Into this morass, enter Jeremy Corbyn. For weeks the leader has been going around the country on a summer tour, taking in more than 50 constituencies – Labour and Tory-held – defying critics who say that he only speaks to the converted.

Read more: Can Jeremy Corbyn win back Scotland for Labour?

The fightback would happen and would succeed, he told a modest rally on Saturday, on a rain-swept park on the edge of Mansfield.

Even in a seat where Labour's incumbent MP was defeated, Corbyn’s magnetic power is still palpable. I was struggling to find the park when I heard the pre-speech chants of “Ohhh Jeremy Corbyn”. There were so many people aching to meet him afterwards that it took a good half-hour to walk the 20 metres from the stage back behind the barriers.

Mansfield would be won back for Labour, Corbyn promised. But in May, the Tory candidate Ben Bradley had said that the town contained a “perfect storm” for his party – and went on to prove himself right by winning.

Mansfield, a market town sustained through the 20th century by the coal mining industry, has had its fair share of problems.

Some in the town and surrounding area feel immigration is too high (net migration in the area was 476 in 2014-15, compared to 1,364 in Boston, where the Leave vote was highest, and 4,598 in Lambeth, where Leave was weakest).

In 2015 Ukip candidate Sid Pepper secured 11,860 votes; he lost all but 2,654 this time round. It was enough for Bradley to win the seat, despite Labour’s share of the vote going up.

This is the kind of place Theresa May had in mind when pledged to protect the "just about managing", with promises of hardline immigration and protectionist economic policies.

May might have come out of the snap election without a majority, but behind the headlines, political analyst John Curtice has found that she did succeed in wooing voters from the C2 and DE income groups.

If the Tories can convince the Leave-voters of the region that they can be trusted to control immigration, their party's hardline position on Brexit could still reap dividends. Talk of defending freedom of movement and immigration into a post-EU Britain goes down well with certain sections of the left. But not in this part of the country – not among the 70 per cent of voters North Nottinghamshire and Derbyshire who voted Leave primarily on the grounds of controlling immigration.

If Mansfield proves anything, it is how precarious the so-called Midlands heartlands could be for Labour. In neighbouring Ashfield, Gloria De Piero’s lead was reduced from 8,820 in 2015 to just 441 in June.

On Saturday, Corbyn was crystal clear that that the fightback had already begun. For the 2,000 who turned up for the rally and braced through the rain, it was clearly just the tonic after such an unsettling election defeat.

Walking back through the town afterwards, on the way to the train station, the rally and the placards and the chanting felt somewhat jarring. (Earlier in the day I’d asked two people how to get to the rally, and neither of them knew it was happening).

This part of the world feels a very long way from Corbyn's own constituency of Islington North. Ukip may be all but disintegrated, but the sentiment remains. Its voters certainly weren’t attracted back to Labour in June.

If Labour is going to win them back, it needs to listen to the very people who don’t turn up to rallies or don’t even know they’re happening. This unassuming region will mean a lot electorally when the time comes. The unthinkable has already happened once, quite easily in fact. Labour can’t afford to let it happen again.