SCOTTISH National Party supporters have spent the last 10 days trying to anaesthetise the shock of the General Election result. “But we won,” they insist, “nothing's changed. Get over it.” They did win in Scotland, of course, delivering more seats in Westminster than all the unionist parties combined. But as with Theresa May's result in the General Election, it didn't seem like any kind of victory. The loss of 21 seats clearly represents a turn of the tide. And in politics, when the tide goes out, that's when we see who's been swimming naked.

The Edinburgh East MP, Tommy Sheppard, writing in the Sunday Herald today, admits that his party just didn’t see Jeremy Corbyn coming, and that the Labour manifesto “dwarfed” both Brexit and indyref2. The SNP is entering a new and difficult era, he says. It can no longer rely on the old strategy of garnering support from Labour voters alienated by a right-wing Blairite establishment. Instead the SNP is in danger, he says, of itself being regarded as the establishment. He calls for the second independence referendum to be “parked” while the party rediscovers a policy agenda that will prove the SNP to be “the real insurgents, the real force for change”.

Sheppard should be congratulated for telling it like it is. But his remarks will upset many nationalists who believe that shelving the referendum risks the entire independence project. The idea that the SNP should outflank Labour on the left will also dismay those nationalists in Conservative constituencies in the north-east and the Borders who don't understand why a party of independence should even be socialist at all. Surely, that is a question to be posed after independence rather than before it?

The SNP hasn't always been a left-wing, pro-European party. It was Alex Salmond in the 1990s who adopted this approach as part of the devolutionary road to independence. After the Iraq war, targeting Labour votes really began to deliver. He and Nicola Sturgeon hoovered up a whole raft of abandoned Labour policies, from abolishing prescription charges to council housing. Noting how damaging Tony Blair's imposition of university tuition fees had been on the youth vote, Salmond promised that “rocks would melt in the sun” before the SNP restored them. Reinventing Labour turned the SNP from a minority party into a party of government after the 2011 landslide.

Salmond, who lost his seat in the General Election, was a unique politician who could bridge class, regional and cultural barriers. He was as at home golfing with bankers as he was canvassing in housing schemes or gassing with farmers at cattle auctions. One minute he could be talking to the CBI in Scotland about cutting corporation tax, and the next winning over metropolitan Guardianistas with his 2012 Hugo Young lecture on the “social wage”. Salmond possessed ideological flexibility because he's an existential nationalist, steeped in Scottish history and culture, who saw national freedom as above party politics.

His successor is not quite so adaptable even though she is probably intellectually superior. At any rate, the General Election showed that Nicola Sturgeon doesn't reach the parts of the electorate that Salmond could, and her popularity has dropped sharply as a result. The First Minister had turned Salmond's tactical opening to the left into the foundation of SNP strategy. She declared herself to be a “utilitarian” nationalist more interested in social justice than in independence for its own sake. She also placed a higher priority on keeping Scotland in the European single market than holding an independence referendum – at least she did until March when she fell into the trap of calling for an early referendum before the Scottish voters were ready to hear it.

The SNP now has to decide whether it moves dramatically left to prevent the haemorrhage of the youth vote to Corbyn Labour, or whether it accepts that this option has now run its course. This will be difficult. For much of the past decade the SNP has been trying to normalise independence, make it sound less radical. When Salmond became First Minister in 2007 he believed, rightly, that the SNP had to be seen as a responsible party of government. The demise of the Scottish Socialist Party, which lost all its MSPs in 2003, was a fate the SNP wished to avoid.

The 2013 independence manifesto was all about not frightening the horses. It proposed what was in reality a form of federalism, or confederalism. Scotland would retain many institutions intact from the unionist era, from sterling to Nato,EU membership to the UK monarchy. The SNP tried to be an establishment party and an insurgency at the same time, and this worked remarkably well – but only so long as the Tory establishment dominated in Westminster. By presenting independence as essentially a social democratic project, rather than an issue of national liberation, the SNP left itself bereft of a response to the sudden emergence of a genuine socialist movement in the UK: Corbyn Labour. Like store-brand cola, Sturgeon was no longer the real thing.

The proposition that the Conservatives could be in government in Westminster in perpetuity now looks faintly ridiculous. However, until a fortnight ago, even Labour MPs believed that the Tories were entrenched and most commentators were forecasting Labour being out of office for at least a decade. What no-one expected was that the insurgency that started in Scotland in 2014 could spread south and transform UK politics as rapidly as it did Scotland during the referendum campaign.

At the Momentum conference in Liverpool last September, I witnessed this at first hand. This eclectic radical Labour tendency, neither Trotskyite nor Blairite, had all the energy and excitement of the Yes Scotland campaign. It was like Women for Independence, the Radical Independence Campaign and National Collective all rolled up in one. This force is now driving the UK Labour Party.

Following the election and the Grenfell fire, Jeremy Corbyn is now the most popular politician in the UK. Nicola Sturgeon looks like yesterday's news, not just to young voters, but to many ex-Labour supporters who had given up on the party under its previous right-wing leadership. Press attention has focused on Ruth Davidson and the Tory revival, but the real story of the General Election result in Scotland is that Labour is now breathing down the SNP's neck in a whole raft of what were thought, even by Labour, to be safe SNP seats. Glasgow East, which looked like one of the safest, is now teetering on a 75-vote majority.

The Common Weal think tank has proposed a series of radical policies including scrapping cuts in Air Passenger Duty in favour of child care; reversing John Swinney's educational reforms and creating a National Investment Bank. These are sensible policies and should be considered in addition to radical land reform and house building. However, the danger is that this might look like a further endorsement of an essentially Labour agenda. Of course, the Corbyn effect may be short-lived. But in the meantime, Nicola Sturgeon may also have to strengthen the argument that independence is a desirable thing in itself, not just a road to social equity, if she wants to prevent the trickle to Labour turning into a flood.