Image copyright AFP Image caption The SNP leader has made her conditions clear for supporting a minority Labour government

This election is happening because of Brexit. For good or for ill, the Parliament of 2017 to 2019 failed to come to a conclusion.

So the still new prime minister decided that the best way for him to achieve his desired departure from the EU was to go again to the country, disappointing Brenda from Bristol, and giving anxiety to the organisers of nativity plays, Christmas fairs, and carol singers - that's even before you start considering the implosion of political operatives looking for ways of running a campaign in the cold and dark of the winter months when, as today's floods are demonstrating, the weather can't be relied on to play ball for balmy summer evenings on the doorstep.

Before the campaign is even a week old, however, it is squarely also about another massive constitutional issue - whether the UK stays as one, or Scotland has another chance to choose to go its own way.

Without obsessing about the potential permutations of a hung Parliament, the polls suggest that neither of the main big tribes are on their way to a thumping majority.

And that means at this stage, it is relevant to think about the what ifs, and the politicians certainly are.

At the SNP's campaign launch today it was striking how explicit First Minister Nicola Sturgeon was about how she might play a powerful hand if there is no overall clear winner on the morning of Friday 13th.

Essentially, she has named her price. Even though she has previously branded Jeremy Corbyn "pitifully ineffective" and "not credible as an alternative prime minister", the SNP leader told us today that she would be willing to work with him on an "issue by issue" basis as long as he gave her an in-principle commitment to holding another referendum on Scottish independence.

Importantly, Ms Sturgeon softened her previous insistence that this would have to happen immediately, suggesting that as long as a potential Corbyn government was willing to commit to letting it happen that would be enough.

Previous Labour leaders have said no way, as plenty of Labour candidates still do. Senior figures like Keir Starmer and Emily Thornberry were quick to say after Ms Sturgeon's comments that there had been no deal, and there won't be.

The 2014 referendum was meant to be a "once in a generation" (remember when politicians used to give those kind of promises?!) decision.

But both Jeremy Corbyn and his right-hand man John McDonnell have given strong signals, that they would not stand in the way of the SNP pushing for another vote, just not in the first year of a potential Labour government.

But we've seen today the two sides may be lining up to hypothetically make it happen if there is a hung Parliament. Ms Sturgeon is not insisting any more it would have to happen straight away and Mr Corbyn is not insisting that it could never happen.

And so, while there are plenty of other issues in this election of course, it's clear one of the possible outcomes leads Scotland again to having the choice of leaving the UK.

Image copyright Reuters Image caption Boris Johnson is hoping talk of another vote in Scotland will drive unionist support to his party

The Tories were already campaigning with claims that somehow the SNP and the Labour Party were in cahoots somehow.

While that was denied, and there is even today nothing official about any arrangement, the First Minister's comments have done the Conservatives a big favour, especially in Scotland where unionist voters who do not like the Tories conceptually one bit, might yet be tempted to vote for them because they don't want another Scottish referendum.

That was a big reason why, in 2017, the Tories surged (relatively!) to 13 seats, and it may help them hang on to more of their seats on the green benches this time round.

And across the UK it gives the Tories more ammunition to back up their accusation that a vote for Jeremy Corbyn could mean more political uncertainty - another referendum on our membership of the EU, and possibly another vote on the UK too.

Of course any of this only comes to pass in the event that neither the Tories nor the Labour Party can form a government on their own. But given the state of the polls, it's a potential scenario, and just as in 2015, the chatter about that potential situation is a factor in the campaign itself, and the SNP are determined to make it so too.

Our messy national struggle over the EU has frustrated politicians and voters alike for three years now. And while some SNP members are wildly enthusiastic about it, there are plenty of people here too who think another referendum on independence would be a disaster.

But in Scotland it's hard to talk about one, without talking about the other. Brexit is the reason for this election. But with no obvious route to a comfortable majority for either Boris Johnson or Jeremy Corbyn, the future shape of the whole UK is now slap bang in the middle of the campaign.