It’s raining in Paisley, great slicing sheets that turn the sky grey. Everyone I meet seems perversely proud of this, as though the bad weather in other places just doesn’t try hard enough. At one point, when a momentary break in the clouds exposes the sun, a Labour activist turns to me and says: “See, people go abroad for this kind of weather.”

The fight for Paisley, Johnstone and the surrounding villages west of Glasgow is a microcosm of the broader election battle in May. Douglas Alexander, the shadow foreign secretary and Labour’s chief campaign strategist, has been the MP here since a by-election in November 1997 (the seat then had slightly different borders). If Ed Miliband ends up in Downing Street, the life of his foreign secretary will be a whirl of red boxes, ministerial Jaguars and negotiations with world leaders.

That’s already one big if. There’s another: Douglas Alexander will be Labour’s foreign secretary only if he gets re-elected as the MP for Paisley and Renfrewshire South.

In any other recent election, that would have been a foregone conclusion. Labour took 60 per cent of the vote here in 2010, with the SNP trailing in second with 18 per cent; the Tories and Lib Dems tied on third with 10 per cent each. Douglas Alexander’s majority was 16,614 – way past “safe seat” and heading into “impregnable”.

Then on 4 February this year those assumptions were turned upside down. The Tory former deputy chairman Michael Ashcroft published a series of polls showing the SNP on course for a landslide in Scotland. The Chief Secretary to the Treasury, the Lib Dem Danny Alexander, was a goner, as were nine of his party colleagues; Gordon Brown’s old seat of Kirkcaldy looked likely to be an SNP gain, along with several constituencies in Glasgow, a city that narrowly voted Yes in last September’s independence referendum. In Paisley, the poll showed that Douglas Alexander’s 60 per cent vote share had fallen to 40 as the SNP surged 8 points ahead of him to 48 per cent.

The same day as Ashcroft released these polls, the SNP’s leader in Westminster, Angus Robertson, announced the party’s latest round of candidate selections. If Ashcroft’s data was right, the next MP for this Paisley seat would be a 20-year-old politics student called Mhairi Black.

In the two months since then, the contest between Black and Alexander has come to feel symbolic of the trends in Scottish and British politics. The SNP would like us to see it as a duel between a “career politician” and the party’s fresh blood, a young woman fired up by the referendum and ready to challenge a broken system. As James Kelly wrote on the nationalist website Bella Caledonia: “This would arguably be the sweetest moment of election night – Labour’s Sultan of Smugness being humbled by a 20-year-old SNP candidate who has been demonised in the unionist press.” The MSP for Paisley, the Scottish Nationalist George Adam, tells me: “Douglas Alexander is a major player in Westminster, let’s not kid ourselves. I may find the fact he doesn’t care if it’s Paisley, Penrith or Perth that he represents – I might find that repulsive, but he is a major player.”

But the fight for Paisley is also illuminating in other ways. The SNP’s ascendancy is inextricably bound up with opposition to a Tory-led government in Westminster and its austerity policies. Where Ed Miliband sometimes seems to oppose George Osborne half-heartedly, in deference to his cautious English voters in Tory-facing seats, Nicola Sturgeon is free to condemn him utterly. It is hard to overestimate the depth of anti-coalition feeling in Scotland: Ashcroft’s polls found that 75 per cent of respondents said they would definitely not vote Tory at the next election, and 73 per cent said they would not vote Lib Dem.

One other figure from that poll should give us pause: 77 per cent said they would definitely not vote Ukip. With a deeper love for Europe, fewer immigrants and fewer terrifying headlines about migrants, Scotland has not been receptive to the overtures of Nigel Farage’s People’s Army. As we went to press, Ukip had not selected a candidate to fight Paisley and Renfrewshire South, and Ashcroft puts the Tories on just 6 per cent. Here, the solution to the growing poverty and inequality that followed the financial crisis is sought on the populist left rather than the populist right.

***

Mhairi Black (“It’s pronounced like ‘Will you marry me?’”) was born in September 1994, by which time Douglas Alexander had already spent time working as a speechwriter for the then shadow trade and industry secretary, Gordon Brown, before returning to Edinburgh to study law. She cannot remember the Major government at all; she is a child of the Blair years, although the economic boom had a muted effect on the area of northern Paisley where she grew up. Her family was not well off, and former schoolmates have struggled with drug addiction and the search for a stable job.

Black’s first vivid political memory is of being taken to march against the Iraq war – chiefly because her aunt gave her a huge cherry lollipop called a “frying pan” that hadn’t been available in Scotland for years. “That’s my memory: marching was magic! But what I did notice was, with Labour it had always been, ‘Oh, they’re the good guys.’ And I remember seeing that change.”

Her father, Alan, now 54, who acts as her unofficial taxi service, had begun to feel disillusioned even before the invasion of Iraq. “The first thing that made him raise his eyebrow was how Tony Blair, one of the first things he did was build the Millennium Dome. He thought: what? We’ve just had however many years of Tory rule, this place is decimated – and you’re building the Millennium Dome?”

Black tells me this over a cup of tea in a Paisley café. Her supporters have abandoned canvassing a local estate, as the rain was ruining the sheets on which the party records voter intentions. On the doorsteps, she mixed wry friendliness with a passionate polemic against austerity and marginalisation of Scotland at Westminster. It wasn’t always an easy sell. One woman informed us: “I don’t like how Nicola Sturgeon’s been going on, just annoying everybody with her attitude.” (Alex Salmond fared even worse: “He’s really a wee plonker.”) But before we left, the prospective voter had conceded Black’s point that decades of backing Labour had not protected Scotland from Tory governments for which it didn’t vote. “I think the English despise us Scots, basically,” the woman said. “You get all these English folks going: ‘Ooh, they’re banging on, ugh, they’re getting too much money.’ Blah blah blah.”

***

As we walked away, Black insisted that this was the first person who had brought up England on the doorstep, but nonetheless this sentiment was a recurring theme of my time in Scotland. There is a distinct lack of sympathy at the thought of the SNP “holding Westminster to ransom” from people who feel Scotland has suffered from successive Tory governments it hasn’t wanted. Once, Labour was the sole vector for opposition to the Right. Under Sturgeon, the SNP's rhetoric has moved decisively left – opposing austerity in a much more full-blooded way than Labour, and delivering its message via a much more popular leader.

Over a hot drink in the backroom of the café, Mhairi Black tells me that her family were always Labour supporters. “My grandpa, he worked on the shipyards in Clydebank – he comes from proper Red Clydeside, socialist roots and all the rest of it. Our whole family’s just been brought up like that from both sides, you know? Even my aunties and uncles: a year ago they were all Labour, and they were all No voters as well.”

That must have made for interesting chats at family occasions, I say. “Eventually, they all swung to Yes. And then the minute it was a No vote, and they seen how Labour were conducting themselves through the referendum, agreeing with the Tories on so many things. . . The whole host of them have joined the SNP now.”

Black’s conversion to the power of politics came during the referendum, when a woman walked into a Yes campaign office with her little boy. “She was dead kind of shifty and that, I thought – she’s nervous, she’s an undecided. I went, “do you want some leaflets or anything?” and she went, ‘Naw, I’m looking for food.’” She sips her drink. “We were scrambling, trying to get stuff together, just teas and biscuits. . . I was just thinking: that’s Victorian.”

If elected, Black could be the youngest MP in the Commons – although she is not the youngest candidate, as Labour are fielding a 19-year-old called Ollie Middleton in Bath. She is a third year student in politics and public policy at the University of Glasgow, with a final exam on 25 May, for which I suspect she is not currently doing much revision.

The biggest threat to her candidacy came on its second day, when the Daily Record found a video of her addressing a Hope over Fear rally in George Square. On the night of the referendum, she told the crowd: “We had to walk past all these fat cat Labour councillors goading us, clapping sarcastically, saying ‘better luck next time’ or ‘hard lines’. It took everything, every fibre in my being, not to put the nut in one of them.” Cue enthusiastic applause.

The newspapers also found posts from her Twitter account – the one she had since she was 14 – which read: “Smirnoff Ice is the drink of the gods - I cannae handle this c*** man” and “I’ve only just realised - I really f***** hate Celtic”. (As one wag commented on a Reddit discussion thread: “To be fair, that does really represent the people of Paisley.”)

Black apologised for the comments, and the SNP stood by her. Now, she says: “I can’t condone anything I said. I look at it and I’m embarrassed by some of the daft things.” But she adds later: “My pal was saying to me: you swore on Twitter; Tony Blair started a war. What bothers folk more? The thing that’s been said most to me is: ach, we all did daft things.”

Nonetheless, the scandal has shaped perceptions of her. It is at the top of her Google results, and one Scottish journalist I mention the constituency to assures me “the Nat is a maniac”. Black is unperturbed, suggesting that it helps her campaign if Labour underestimate her. “That’s their mistake: they’ve got used to power and they’ve taken it for granted, and then when you’ve got anybody making any kind of argument, that’s when the patronising judgements come out. You’re just a daft woman. You’re just a wee lassie, you don’t know what you’re talking about.”

Does she think she’s going to win – or is Scottish Labour facing a version of the “Shy Tory” affliction, where its voters are reluctant to out themselves to pollsters? “The obvious answer is I don’t know. I don’t. But don’t get me wrong, it’s brilliant being ahead in the polls . . .what is definitely coming across is that people are hungry for change.”

***

It is not raining in Johnstone, the next largest town in the constituency, on the following day. It is hailing. Combined with the high winds, this is like having handfuls of gravel thrown into your face. Again, this doesn’t seem to be denting anyone’s enthusiasm for being outside: as I get out of the car, Labour councillor Iain McMillan starts singing “Jingle Bells”.

As we tour Ulundi Road and Hagg Cres with bags of leaflets, Douglas Alexander doesn’t look like a man who has given up hope of winning the seat. He is in good spirits following the release of Labour’s latest party political broadcast, starring Sherlock actor Martin Freeman. The day after its release, the simple straight-to-camera piece is well on its way to a million views.

Alexander dashes round the houses without hat or gloves, a puffa jacket over his workwear, sprinting off at one point to say hello to a woman who worked with his mother at the local hospital. It’s the early afternoon, and many of the retired residents are at home, although no one seems to want to argue politics on the doorstep.

The operation hums with easy smoothness, with former Better Together director of operations Kate Watson acting as air traffic controller of the dozen activists’ movements. She does this with an increasingly soggy clipboard; iPads are one Obama campaign innovation that has not yet made it to British politics, despite Labour’s hiring of American guru David Axelrod. Back at party headquarters, all the data collected on the doorsteps will be fed into Labour’s specialist software Contact Creator, a key part of its get out the vote operation. One of the party staffers tells me that the software suggests the SNP lead is nothing like as solid as the Ashcroft poll suggests.

In the past, Labour could rely on its greater organisational strength and activist base to dominate Scottish politics, but that has changed as SNP membership soared in the wake of the referendum. The nationalist party now claims 105,000 members, with 2,022 joining during the seven-way leaders’ debate on April 2. By contrast, Jim Murphy suggested Scottish Labour had “about 20,000 or so” members in December. (The party claims 190,000 nationwide.)

I suggest to Douglas Alexander over tea in Papamacs deli that he has another disadvantage. The SNP has a compelling narrative for this campaign: at 20, Mhairi Black has become a cipher for a new kind of politics, against which he can be painted as the old, discredited establishment. What is his counter-narrative? How does he see this election? “Renfrewshire needs to get rid of the Conservative government, and get changes that Labour can offer. An end to zero-hours contracts, more nurses for our local hospital, the changes that working people need. The real risk would be to see the Conservatives back in office after me, and not secure the practical changes that people want.”

As Labour’s chief election strategist, Alexander is obviously a professional politician – in both the positive and negative senses of the word. We don’t complain if plumbers or brain surgeons are “professional”, after all – but equally, it’s possible to see how his landmine-tested answers can seem less authentic than the youthful chattiness of his opponent. Unlike Mhairi Black, he pays close attention to what can be said “in front of your tape recorder” – but then he wouldn’t have survived for more than a decade at the top of politics if he put too trust in journalists. Eileen McCartin, the Liberal Democrat candidate who first ran against him in 1997, tells me over the phone: “In a personal sense, he is always a gentleman. Like most Labour politicians, he toes the party line and says what needs to be said for his party.”

Alexander admits that “people are pretty scunnered with politics” and draws attention to what he sees as the failures of the government in Holyrood – run by the SNP as a minority administration since 2007, and with a majority since 2011 – in handling devolved healthcare. He argues that far from offering an end to austerity, the fiscal autonomy proposed by the Nationalists – and the consequent end of the Barnett formula – would result in spending cuts. “The SNP’s economics are all over the place.”

There have been suggestions that Alexander would have preferred to run a less overtly left-wing general election campaign, with one friend telling the Financial Times: “Douglas knows in his heart that the most effective way of winning for Labour is the Blairite way, but he will run the campaign Ed’s way.” Yet the 47-year-old seems genuinely passionate when he talks about tackling poverty in his constituency. The Renfrewshire foodbank is one of the five busiest in Scotland, according to the Trussell Trust, feeding 6,000 people a year.

On Sunday 29 March, as the SNP were holding their spring conference, Alexander had written a piece for the Scotsman which argued that Labour’s “vision for the common good is one that ends the need for food banks”. But there are two in your constituency, I say. “I helped established them, with the local churches,” Alexander shoots back. “I’ve never believed in revolutionary impoverishment. If my neighbours are in need, then I feel it’s right that we work together to help them. But it’s a moral obscenity in the twenty-first century that a community like Renfrewshire is having to rely on foodbanks.” (Ewan Gurr from the Trussell Trust disputes this account: “He came along, he cut the ribbon, he said some fantastic words about the fact his dad had been a Church of Scotland minister and had been involved in providing food for people as well. Douglas is a good guy, I think he's got a good heart, but I think it's a bit disingenous to say he was involved in the set-up of Renfrewshire foodbank. That is factually inaccurate.")

Alexander sees the battle between Labour and the SNP as a contest between solidarity and division. “I’ve contested elections against the SNP for 20 years, so in that sense I know what the SNP are and I know what they represent,” he says. Which is? “It’s a politics of identity rather than a politics of ideas. . . My politics isn’t premised on a sense of grievance and other, which is at the heart of a lot of nationalism.” He is dismissive of Alex Salmond’s grandstanding about the SNP’s importance in a hung parliament. “He’s in the business of suggesting that the way to get a Labour government is to vote SNP. Alex Salmond said in 2010 to voters in England: ‘vote Liberal Democrat’. Nicola Sturgeon said last month to voters in England: ‘vote Green’. They seem to have an ‘anything but Labour’ strategy in order to get a Labour government, and that doesn’t make sense to me.”

His next sentence foreshadows the Telegraph’s claim, on 3 April, that Nicola Sturgeon told the French ambassador she wants David Cameron to remain in Downing Street (a claim she denies). “Salmond and Sturgeon are both trying to drive down the Labour vote in Scotland and drive up the Tory vote in England. Why? Because as Alex Salmond said just last week, he wants Scotland to have a “second chance”. This is a man who said, just last year, this was a ‘once in a lifetime opportunity’.”

Alexander admits that many people he speaks to on the doorstep see the general election as a chance to “re-litigate” the referendum, and they are very hard to persuade away from the SNP. But he still thinks he will win: “I’m confident, but not complacent. My majority in 1997 was about 2,700 and it’s increased at every election since then. That’s because I have never taken this community or this electorate for granted. I never have, and I never will – because they’re the people I grew up with.”

This is the view also taken by the Lib Dems’ Eileen McCartin. When I ask her who she would bet on to win the seat, she laughs. “I'm not a betting person, but I have said publicly that I don't think the SNP will take the seat from Labour. I wish it were the Liberal Democrats that were winning the seat! I don't think the SNP are nearly as strong in the country as it would be suggested, and I have my doubts.”

As saccharine as it sounds, from what I could see, the fracturing of British politics has been good for anyone in Paisley and Renfrewshire South who thinks of herself as progressive. The constituency has a choice between two principled, committed politicians vying to represent it at Westminster, campaigning on a platform of fairness in the welfare system and greater equality. And whether its next MP is Douglas Alexander or Mhairi Black, I doubt either of them will take their victory for granted.