Nicola Sturgeon by Ralph Steadman

Activists rush in and out of Natalie McGarry’s campaign office on Westmuir Street in Parkhead, a short walk from the towering grey-and-green stadium of Celtic Football Club. The Scottish National Party candidate for Glasgow East, an energetic 33-year-old policy officer who rose to prominence during last year’s independence referendum, is preparing her team for the first of its twice-daily canvassing sessions. “OK, let’s go,” she says. “We’ve got a lot of work to do.” If on 7 May the Nationalists can win here, where the sitting Labour MP, Margaret Curran, has a majority of almost 12,000, they will sweep the country, defeating Labour in its west coast and central belt heartlands.

In Glasgow East, as in Scotland at large, ideology, identity and class have merged to shape a new political landscape. Left-leaning voters, voters who consider themselves strongly Scottish and voters from low-income or working-class backgrounds account for a large section of the SNP’s expanding post-referendum base. According to a recent survey by YouGov, 40 per cent of Scots who backed Labour at the 2010 general election now support the SNP. A similar proportion of Labour supporters voted Yes on 18 September.

“People think Labour has taken them for granted,” McGarry tells me as she traverses the doorsteps of Easterhouse, a dilapidated estate on the outskirts of the constituency. “The amount of inequality in this area is frankly disgusting, yet Labour has stacked people’s votes for generations.” McGarry is accompanied by a group of six canvassers, including three who joined the SNP as a result of the referendum, defecting from Labour and the Scottish Socialist Party. The other three are SNP veterans. No one in the group is surprised that McGarry is the favourite to win the seat. They believe Labour’s decline is long overdue. “Our canvassing returns are really encouraging,” McGarry says. “But we’re taking nothing for granted.”

As the SNP quietly readies itself for victory, the mood among Scottish Labour supporters is grim. When Jim Murphy was elected party leader in December, he made an audacious attempt to claw back “Glasgow Man”, the archetypal Labour-to-SNP switch voter, by reinventing himself from a Blairite unionist to a home-rule social democrat. But in recent weeks, as the SNP’s poll lead has solidified, Murphy has abandoned this strategy and appealed instead to the anti-nationalist core vote. During the Scottish TV debates in early April, Murphy attacked the SNP leader Nicola Sturgeon’s plan for full fiscal autonomy and criticised her refusal to rule out a second referendum on independence.

Some Scottish Labour insiders fear that Murphy’s decision to court No voters with a volley of tribal broadsides against the SNP will harden referendum loyalties, pushing “Glasgow Man” further into the arms of the Nationalists. “Instead of encouraging voters to move on from the referendum, Murphy seems intent on refighting it with the same tactics as before,” a senior Scottish Labour activist tells me. “But this time around, 45 per cent of the vote will deliver the SNP almost every Labour seat in Scotland.” At this late stage in the campaign, however, Murphy’s options are limited. In the seven months since Scotland voted on independence, rejecting it by a margin of more than 10 points, nothing else has worked to halt, or even slow, the SNP’s ascendancy.

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Alex Salmond stands in the middle of the Balmacassie Industrial Estate in Ellon, Aberdeenshire, surrounded by employees of the BrewDog headquarters and distillery. After a fire alarm sounded and workers gathered in the car park, the former SNP leader seized the chance to stage an impromptu photo opportunity. Although dressed casually in tartan golf slacks and a blue polo shirt, Salmond is in full campaign mode. Next week he will find out if his ­attempt to replace the outgoing Liberal Democrat MP for Gordon, Malcolm Bruce, has been a success.

The battle for Gordon highlights an ­under-reported element of the SNP surge: the collapse of the Liberal Democrats in Scotland. Support for Nick Clegg’s party may have rallied a little in England but it ­remains terminally low north of the border. Polls suggest that the Lib Dems could lose ten of their 11 Scottish seats, including Gordon, to the SNP, with just Orkney and Shetland, a traditional Liberal stronghold, left standing.

Following a tour of the plant – Salmond wanted to meet here because he views BrewDog, a producer of high-quality craft beer, as a great Scottish success story – I sit down with the ex-first minister. He is in bullish form. “I’m very confident,” he says of his prospects in Gordon. “I’ve never lost a parliamentary contest. And the reason I’ve never lost is because I take absolutely nothing for granted. I work like fury.”

Salmond is unconcerned by reports that tactical voting by supporters of the Union could upset his return to the Commons. “Tactical voting is the philosophy of ­despair,” he says. “‘Don’t vote for me because you like me, vote against somebody else because you don’t like them.’ It’s pathetic. People vote for things, in the main. Politics is about a positive vision.”

Throughout his second stint in charge of the SNP, between 2004 and 2014, Salmond instilled a sense of confidence in the party that has helped steady it during moments of crisis. In retrospect, his decision, announced on 19 September, the day after the independence referendum, to resign and carry the responsibility for defeat with him, was a smart tactical move. His departure allowed his successor, Nicola Sturgeon, to concentrate on the forthcoming election.

Alex Salmond by Ralph Steadman

But now Salmond has to carve out a new role for himself in the party. Assuming he wins in Gordon, where does he fit into the SNP set-up at Westminster?

“I’m sure I’ll have a part to play in the ­parliamentary group and I’m perfectly content with that. But this is Nicola’s campaign,” he says. “She’s the leader. You can’t have two campaigns and you can’t have two leaders.”

Salmond praises the performance of Sturgeon, whom he claims has taken the SNP to new heights of popularity in England as well as in Scotland. “I think the real people of England, the plain people of England, the ones who have not spoken yet, seem to be taking to Nicola with tremendous enthusiasm. Otherwise we wouldn’t have all these people writing to us asking, ‘Why can’t we vote for the SNP in Slough?’”

Yet on the question of a second independence referendum, Salmond’s position differs from that of Sturgeon. While the First Minister argues that a British exit from the European Union could trigger another poll in Scotland, Salmond cites the failure of the UK parties to grant Scotland home rule as a potential catalyst. “We want the implementation of the Vow,” he says, referring to the promise of additional powers unionist leaders made on the eve of the referendum. “Not the watered-down, anaemic, diluted version of [the cross-party Smith Commission report] but ‘devo-to-the-max’, to quote the Daily Record; ‘as near to federalism [as it is possible to get within the UK]’, to quote Gordon Brown – or home rule, to quote everybody.”

Independence remains the SNP’s lodestar and motivating principle, but some Nationalists think that the party isn’t pursuing it rigorously enough. During her speech at the launch of the SNP manifesto on 20 April in Ratho, a small town just outside Edinburgh, Sturgeon received huge applause for reiterating her commitment to Scottish self-determination. “The SNP will always support independence,” she said, then added: “. . . but this election is not about independence. It’s about making Scotland stronger.”

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As far back as January, the SNP leadership decided to base its election campaign around three core themes: more autonomy for the Scottish Parliament, an end to austerity and opposition to the renewal of the Trident nuclear missile system. The goal was to outflank Labour on the left and consolidate SNP support among Labour voters who had switched ­after the referendum.

The strategy is working but the SNP could still find itself marginalised at Westminster after 7 May. Having categorically ruled out a deal with David Cameron and the Conservative Party, the Nationalists will have little leverage over a minority Labour government. Indeed, on the issue of Trident, SNP strategists privately concede that their negotiating position is weak. With Tory support, a minority Labour administration will comfortably win any vote on renewal.

Nonetheless, the SNP seems determined to prevent the Tories from forming another government. “There are parliamentary procedures that can be used to secure a non-Tory majority and we are prepared to use them,” says Angus Robertson, the SNP’s general election campaign director and leader in Westminster. “It will depend on the arithmetic in the Commons and whether Labour is prepared to join with the SNP to keep the Tories out.”

What if the Tories emerge from the election with a ten- or 20-seat advantage over Labour? “The UK is not a presidential democracy,” Robertson says. “Governments need to have majority support in the ­Commons. If the electorate decides there should be an anti-Tory majority at Westminster, politicians should act on that.”

Some commentators accuse the SNP of trying to manufacture a crisis of legitimacy at Westminster, or of plotting to stoke anti-Scottish resentment among English voters, to advance its goal of independence. “[Sturgeon’s] plan is as simple as it is deadly,” Matthew Parris wrote in the Times on 18 April. “Run a minority Labour government ragged, kick it around, wreck its authority – but refuse to let it die.”

This view is shared by Gordon Brown, who, during a rare campaign appearance in Fife on 23 April, described the prospect of a Labour administration held aloft by SNP votes as “a match made in hell”.

However, in all probability, the SNP is playing a more subtle, and less Anglocentric, game. Nicola Sturgeon can make life difficult for Labour in Scotland by working with Ed Miliband to implement progressive reform across the UK. By supporting left-wing Labour policies such as a mansion tax and a bank levy (both of which feature prominently in the SNP manifesto), she can occupy the social-democratic centre ground of Scottish politics as the 2016 devolved elections approach. Moreover, like Salmond, Sturgeon comes from the gradualist wing of her party. She views competent government as crucial to building Scottish self-confidence and, ultimately, to achieving independence.

On this point, Sturgeon is emphatic. “I firmly believe that success begets success,” she told me in a brief email exchange from the campaign trail. “People used to say the SNP [wanted to wreck] a devolved parliament. But our record, both in opposition and in government, shows we have done everything we can to make Holyrood a success. This was part of the process of increasing support for independence to 45 per cent from around 30 per cent.

“Similarly, helping to deliver progressive policies at Westminster would enhance the party’s credibility still further, and stand us in extremely good stead for the opportunities ahead.”

Since she became leader in November, Sturgeon has given the SNP a more coherent left-of-centre profile. In recent months, she has abandoned Salmond’s flagship pledge to cut the Scottish rate of corporation tax, committed to reintro­ducing the 50p top rate of income tax and rolled out an ambitious programme of land reform. Under her direction, the SNP is becoming Scotland’s natural party of social democracy.

The effect on Scottish Labour, already weakened by defeat at successive Holyrood elections in 2007 and 2011, has been ruinous. The party’s credibility in once rock-solid constituencies such as Glasgow East is evaporating. After the referendum, Jim Murphy tried to strengthen Scottish Labour’s patriotic and working-class credentials. In January, he announced plans to use the revenue generated by Miliband’s proposed mansion tax, the bulk of which would come from properties in London and the south-east of England, to hire 1,000 more Scottish nurses. He has also sought to remind voters that the SNP is, and has been for the past eight years, a party of government, not an anti-establishment insurgency. But little seems to stick.

On 7 May, the SNP will have an opportunity to inflict lasting damage on Scottish Labour, the linchpin of unionism in Scotland. If it succeeds, Ed Miliband may still end up as prime minister – but of a badly fractured United Kingdom.