Before the Easter break, Scottish Labour activists were shuffling their leaflets for the local government elections, and hoping that they could somehow steer the conversation on the doorstep away from constitutional matters and onto the falling standards in education and the NHS. In this, they seemed to have an unlikely ally in Theresa May.

The British Prime Minister had quashed the immediate threat of a second independence referendum. When I met with pro-union campaigners shortly before Easter, there was a quiet hope that the prospect could be shelved until after the Brexit negotiations - in other words, the 2020s. Meanwhile, the Scottish public might have woken up to the fact the SNP-led Scottish government hadn't actually passed any legislation in a year.

Then the nightmare happened. The pro-union left returned from eating chocolate eggs to find May on the steps of Downing Street announcing an early election on 8 June 2017.

“I think we would regard it as a good day if we at least hold onto one seat,” a Scottish Labour activist told me. That seat is Edinburgh South, held by Ian Murray, the sole survivor of the calamitous 2015 general election. It is viewed as a three-way marginal – Murray’s majority over his SNP rival was just 2,637 votes, and the Tories are also on the rise.

If that wasn’t bad enough for Scottish Labour, the constitutional debate that sank it in 2015 is allowing both the SNP and Tories to thrive.

“Clearly we are starting from a tough place in Scotland,” said Duncan Hothersall, the editor of Labour Hame. “With the SNP and Tories already framing this as yet another nationalism versus unionism vote we will struggle to find the space to be heard.”

Labour, he says, must be “pro unity” and “reject nationalism in its Scottish and English varieties”. Other activists tell me it will ramp up its message of poor SNP governance.

Labour’s brand of progressive unionism is being eclipsed by the tank-straddling Davidson. An Ipsos Mori poll found that 48 per cent of Scots think May is doing a good job, and 53 per cent approve of Scottish Conservative leader Ruth Davidson, compared to 42 per cent for Scotland’s SNP First Minister Nicola Sturgeon, and 36 per cent for Scottish Labour leader Kezia Dugdale.

On the face of it, this means that an early election could be a boon for pro-unionists. The problem is that even while Davidson is widely admired by her opponents, she is seen as too divisive. Even before a second referendum was officially on the cards, the head strategist of Better Together told me "she is not a figure everyone would unite around".

Where progressive unionists are optimistic, it is often for counterintuitive reasons. One suggested a larger Conservative majority could allow May to pursue a softer Brexit, which would placate Scottish voters. Another theory is that May could do Labour a favour by forcing Jeremy Corbyn's resignation and inadvertently reinvigorating Labour. If Davidson did an SNP and pulled off an unexpected landslide, or split the country down the middle, it would put paid to the SNP's claims to speak for Scotland.

The SNP knows this. “Nicola Sturgeon and Ruth Davidson have nailed their colours to the mast,” a source close to the pro-union movement told me. “This is going to be a proxy for a referendum.” He added: “It is pretty hard to see how Nicola Sturgeon doesn’t win this.”

While pro-union sentiment is passionate, and widely held responsible for the Conservative surge, the SNP gains energy from fights with Westminster. It has consistently argued that it is the voice of Scotland’s Remain majority, a message that appears to resonate with voters. In the First Past the Post system, its candidates also benefit from the fact the pro-union vote is split between several parties. For all Davidson’s popularity, Labour activists on the ground expect the Tories to make a dent in the SNP’s majority, but not a devastating one (Davidson herself has reacted to the early election by saying she is optimistic about "increasing our number of seats".

However, the pro-union campaigner feared an aggressive campaign by Davidson will be matched by a similarly polemical response from the SNP, and both will brush the scars of the 2014 referendum. “What happens the day after?” he wondered. He believed Scotland was a factor in May’s decision to call the early election, but not the decisive one.

“Theresa May is thinking about 12 different fronts,” he warned. “It is pretty hard to see how Nicola Sturgeon just concentrating on this one thing doesn’t find a way to turn it to her advantage.”

Which SNP MPs could be under threat?

Name: Michelle Thomson

Constituency: Edinburgh West

Majority in 2015: 3,210

Michelle Thomson is officially an independent MP, having withdrawn from the SNP whip over a police investigation into her property deals. The Liberal Democrats came second in 2015 and will find it hard to resist stirring up trouble for Thomson again.

Name: Calum Kerr

Constituency: Berwickshire, Roxburgh and Selkirk

Majority in 2015: 5,901

Calum Kerr may have won his seat in the Scottish Borders by more than 5,000 votes, but the runner up was a Tory, John Lamont, who went on to win the Scottish Parliament seat in the same area a year later.