Siôn Simon, Labour’s candidate for the new job of West Midlands mayor, is out canvassing on a sunny day in the Black Country, trying to persuade residents to pick him as their choice for what he calls “a job that doesn’t yet exist”.

Hamid and Basit Miah are Labour fans. They have invited Simon into their home and they perch smilingly alongside him on the sofa as the enticing smell of cooking and the sound of a child playing drift from a back room. “We support Labour, we’re Labour all the time, we have always been for Labour,” says Hamid.

Simon thanks them and reminds them to vote on 4 May. Then, after a pause, Hamid asks: “Are you backing Jeremy Corbyn? Are you? Really? Because I have my doubts with Jeremy.

“We need to have a leader who knows to stand up. There’s no unity there and it’s just a shame. It’s sad.”

This area is solidly Labour. Some residents emerge from their houses to shake hands and take pictures with Simon, and with the familiar figures of John Spellar, the local MP, and Tom Watson, Labour’s deputy leader, who are canvassing alongside the mayoral candidate.

Simon insists he is not interested in Corbyn and his future, dismissing it as a Westminster preoccupation. “I just don’t get into that conversation, because it is not what this election is about,” he says. “At the same time he got elected as leader of the party, I got elected as the party’s candidate for the West Midlands with 72% of the vote, so a lot of people who voted for him, voted for me. And a lot of people who didn’t vote for him, voted for me.”

Simon in the home of two Smethwick residents. Photograph: David Sillitoe/The Guardian

But if Simon fails to win, in an area that has long been a Labour fiefdom, it will inevitably be seen as a barometer of the party’s ability to hold on to heartland seats – and of the extent to which the Brexit vote has shifted Britain’s political landscape. The new mayor’s patch will take in seven council areas, including Dudley, Walsall and Wolverhampton, as well as Birmingham and Coventry.

Some of the focus in May’s local elections will be on the byelection in Gerald Kaufman’s old seat of Gorton, in Greater Manchester, and the more high-profile races for metropolitan mayors in Liverpool and Manchester.

But it is the West Midlands race that many Conservatives will be watching most closely. It is a tempting prize for Theresa May’s party. Birmingham, which now bristles with glass and steel high-rises and posh shops, was the stomping ground of Joseph Chamberlain, who turned civic power here into national statesmanship and is a hero to one of May’s key advisers, Nick Timothy.

Winning here would plant a flag firmly in Labour territory, providing perhaps an even stronger signal than Labour’s loss of the Copeland byelection in February that the Tories can penetrate parts of the country long considered beyond their reach. Elections expert John Curtice calculates the Conservatives would need a significant 12% swing to take the seat, but believes it is likely to be very close. Picking up the second preferences of minor parties’ backers will also be important, as the poll is being carried out using the supplementary vote system.

The Conservative candidate in the West Midlands, former John Lewis chief executive Andy Street, was given the star treatment at the party’s annual conference in Birmingham in October, and has received a string of visits from senior ministers.

Street says: “This would be a major achievement for the Conservative party, because it really would show that for the first time in perhaps 30 years, with the exception of London, the Conservatives can win in urban Britain, and where one-third of the population is BME [black and minority ethnic]. That would be a very dramatic statement on behalf of the Conservative party.”

Many grassroots services will remain with local councils, but the mayor will be backed by a new combined authority, which is already in place and will take oversight of issues including housing, transport and adult skills. As part of the devolution process set in train by the former chancellor, George Osborne, the region is set to receive more than £30m a year from central government for the next 30 years towards an investment fund. It will take charge of a series of funding streams currently spent from Whitehall.

Simon talking to a Smethwick resident, along with Tom Watson, deputy leader of the Labour party. Photograph: David Sillitoe/The Guardian

Some of Simon’s manifesto promises are about the nitty-gritty of infrastructure, such as nationalising the M6 toll road and building a new runway at Birmingham airport. But he argues the iterative nature of devolution should allow whoever wins to fight hard for more powers over time. “The legislation that underpins the whole thing is explicitly and deliberately dynamic – it’s set up as a process,” he says.

He would also lobby for more generous funding for the region, calling for the abandonment of the Barnett formula, which allocates central government cash to the regions and nations of the UK in a way that Simon claims systematically advantages Scotland, Wales and London.

“Public expenditure per head in London, Scotland, Wales and Northern Ireland is £10,000, in some cases a lot more than £10,000. In England as a whole it’s £8,816 and in the West Midlands it’s even less than that. The truth is that if you go to Scotland, they’ve got better schools and parks and public services than we have. This is not some arcane accounting trick; this is 40 years of cumulatively extra investment in public services,” he says. “I think that conversation is one that can’t be ignored.”

Simon has been waiting a long time for this chance. A junior minister in the last Labour government, he stepped down from parliament in 2010 and threw his energies into campaigning for a Birmingham mayor. That idea was rejected at a local referendum in 2012, but the latest wave of devolution deals has created the post of mayor for the whole of the West Midlands.

Simon says part of his job would be celebrating a coherent West Midlands identity. “We have a common heritage,” he says. “It’s a manufacturing region, it’s an engineering region, it’s a creative region. This is the region of Shakespeare and George Eliot, the finest English novelist; Elgar, the quintessential English composer; Tolkien, who’s the only one of those four who’s a Brummie. People who live here don’t even realise that. We don’t tell that story ourselves.”

He has the backing of Watson and Spellar, but the Unite union, Labour’s biggest financial backer, withheld £10,000 of funding from Simon’s campaign – despite supporting him for the nomination in the first place – in a row over a proxy battle for control of the party.

Len McCluskey, who is standing to retain the leadership of Unite, recently accused Simon and Watson of providing covert support for his rival, Gerard Coyne, in what he called “a low-budget remake of The Godfather”.

Voters in Smethwick do not appear to be concerned about the West Midlands mafia, but some at least have picked up the sense of a party not at ease with itself. When turnout is crucial, that may yet matter.

“The Labour vote in Labour areas, which is most of the West Midlands, is holding up. It’s not the best it’s ever been, but we’re winning – but by a lot less than we usually do,” Simon says.

And in response to Hamid Miah, fretting about whether Labour is providing robust enough opposition, Simon says: “Please, let’s not be concerned about which Westminster politician is doing what, because that’s not what this is about.”