A self-confessed political “geek” and football obsessive, Mhairi Black – like any university student – is happier in jeans than in the smart business suits she is having to get used to wearing.

“My mum insisted,” said the 20-year-old. “We went shopping the minute I was selected.”

In the concrete car park directly across from the ancient abbey of Paisley – resting place of the royal house of Stuart and a place where 19th-century radicals once plotted an overthrow of the government – Black’s supporters are more warmly dressed in Puffa jackets and scarves. The evening sunshine is giving way to drizzle and a chilly wind. Black must be freezing as she takes out her Scottish National party rosette and pins it to her lapel, but beams warmly. “Right, at least its drier than last night. So where are we going?”

If the pollsters are accurate, Black could be going all the way to Westminster. According to a February poll by Lord Ashcroft, Black is 8% ahead of her rival. Victory would be a remarkable feat and make the SNP candidate for Paisley and Renfrewshire South the youngest ever MP, as well as overturn the 16,610 majority of one of Labour’s biggest hitters in Scotland, shadow foreign secretary Douglas Alexander.

Referring to its radical traditions, Benjamin Disraeli once said: “Keep your eye on Paisley.” Students of political upsets would do well to heed the advice on 7 May.

As Black canvasses round an estate of modern terraced houses in central Paisley, a town some seven miles west of Glasgow, everyone who answers their door has an opinion. “You’re kind of young to be taking on a big gun,” said Joseph Findlay, a lifelong Labour voter. But he’s prepared to listen and takes the literature he’s offered. “I think the SNP will win round here,” he said. “As long as David Cameron is put out, that’s the main thing.” Findlay is the only one who mentions her age, but everyone has something to say – about welfare, the NHS, dirty pavements, tax, even euthanasia.

Though slightly startled by her own trajectory, Black is not remotely surprised by the SNP surge. “People have been battered by a system that doesn’t care about them and they’ve had enough of austerity,” she said. “You can feel the frustration that people have, that the principles have been abandoned by Labour. My dad is a lifelong Labour voter: my family was all proper Clydeside-shipyards Labour. But my grandfather would turn in his grave if he heard how Labour were behaving now: the bitterness in them. The intellect of the party has fallen, the principles of the party have fallen. They started taking people for granted. My dad and I joined the SNP on the same day. Labour have never had to campaign here, so I wanted to stand to force Douglas Alexander to do just that.”

Alexander, in charge of Labour’s national election strategy, knows he is in a battle now. The membership of the SNP has tripled in the past six months to more than 106,000 – the party claims 1,200 people signed up during Thursday’s television leadership debate in which its leader, Nicola Sturgeon, starred. It has become the third largest party by membership in Britain.

For Black, the legacy of Scotland’s in/out referendum is a renewed political energy and determination, and a growing self-confidence among those who want Scotland to shape its own destiny. “It took a day or two to lick my wounds, and then I just thought: right, what can I do now? And a lot of people were like that. People had a confidence, that capability. You knew that you could be a painter and decorator and know more about what needs to be done in this society than anyone in a suit in Westminster. Because you know what it is to be skint and to live with the anxieties of that in your life. That’s what the Labour party used to stand for. We don’t need to be millionaires, we just want to be making a decent living and having our views listened to.

“It’s about making a noise, whether writing to your MP or going on a march. I’ve got a lot of friends who live in England who are really jealous of what’s happening in Scotland, who would love to be part of something like this.”

My grandfather would turn in his grave if he heard how Labour were behaving now: the bitterness in them Mhairi Black

Black tackled the selection process in the way she is tackling the election: singlemindedly. The first girl in her primary school to be selected for the football team, she clearly has determination and drive.

But it was just a day after her selection that the first hard political knock came. In what seems an oversight by the SNP PR machine, Black’s Twitter account still held tweets from when she was a teenager. During an especially tense match between her beloved Partick Thistle and rivals Celtic, she had tweeted: “I’ve only just realised – I really f***** hate Celtic” and “Celtic, yer a joke! #scum.”

“Yes, making the front page was a surprise,” she said ruefully. “My brother came through and said: ‘Have you seen this?’ But then he said: ‘You got a bit heated during a football match. Tony Blair started a war. Let’s put it in perspective.’

“So it was a shock to the system, but you have to think about what your political opponents are doing, trawling through to find old messages written by a teenager and holding them relevant in a country where 100,000 children are living below the poverty line. Maybe their priorities are a bit skewed.”

Douglas Alexander, the shadow foreign secretary, outside Paisley Abbey. Photograph: Murdo MacLeod

That wasn’t the end of the controversy. At a post-referendum rally, Black described being at the count and walking past “all these fatcat 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.” Unfortunately for her, the cameras were rolling.

“Of course, I regret that turn of phrase,” she says now. “That day, after two years of work, of course it was emotional. And no, I wasn’t about to headbutt anyone.”

On his doorstep, Paisley voter Joseph Findlay is impressed with Black. He’s never before voted SNP, but he’s now considering it, disillusioned with Labour. “I think they judged it wrong, they scared people and that’s not right. There’s a lot regretting it now.

“I voted Labour all my life, my wife voted no in the referendum, and I voted yes. She’s riddled with guilt now she sees that nothing has changed. We’re both not decided yet for the general election but I think the SNP are going to win around here.

“Us oldyins haven’t done much to put up a fight, so now the younger ones are getting out and doing it for themselves.”

He peers round his doorframe at the departing figure of Black, off to “chap” his neighbour’s door. “Very good luck to the lassie.”

If the directness of the voters around here is anything to go by, there is no tolerance for the status quo. Alexander may yet pull back some of his party’s deserters before the vote, but there is a new wave of post-referendum politicised Scots who won’t make it easy.

Black refuses to even think about losing. “I’m fighting this election day by day, talking to as many people as I possibly can. I want to change things, that’s what people are demanding. You can feel it bubbling – the frustration and the anger.”