Rarely can a country’s politics have been defined so clearly by a debate about referendums.

At one end of the spectrum, the SNP and the Greens want two, one on Brexit and one on independence. At the other end the Conservatives, UKIP and the Brexit Party do not want a referendum on either subject.

In between are, first, the Liberal Democrats (and Change UK) who would like another referendum on Brexit but not on independence and, second, the Labour party who are clear they oppose another ballot on independence but are neither definitely for or against another Brexit ballot.

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Consequently, whatever may be the position elsewhere in the UK, in Scotland the Euro-elections will be about more than Brexit. They will also be the occasion for another round in Scotland’s continuing constitutional debate, especially given that last week the First Minister re-affirmed her wish to hold another independence referendum sooner rather than later. But what will be the impact of this double fault line?

Are not all of the parties at risk of chasing no more than relatively small niche markets? After all, in the last Euro-elections in 2014, held before the UK’s membership of the EU had become such a live issue (as well as before the independence referendum), no party won as much as 30% of the vote and only the SNP and Labour secured more than 20%.

The potentially cross-cutting nature of the two debates was very clear at the time of the EU referendum. Even though the SNP’s vision of independence has for nearly thirty years been one of independence inside the European Union, as many as one in three of those who voted Yes in 2014 backed Leave 2016. 

The unionist camp was divided on Brexit too, with just over two-fifth of those who voted No backing Leave. No single combination of independence and Brexit support represented more than 30% of voters.

Yet two polls published at the weekend both suggested that the SNP was heading for as much as 40% or so of the vote, more or less in line with its current popularity for both Westminster and Holyrood. Rather than simply winning much more than a niche market, the party seems set for its best performance yet in a Euro-election.

The explanation lies in the fact that, these days, the SNP not only dominates among Yes voters in a way that it never did before the 2014 but also has become very much the most favoured party among Remainers north of the border. Thus, while as many as two-thirds of those who voted Yes in 2014 say they will vote for the SNP next month, so also do almost a half of those who voted Remain.

Either pattern on its own is enough to secure the SNP at least 30% of the vote. Meanwhile, not every voter fits the general pattern and, despite the party’s stance on Brexit and independence, around one on five Leave voters and about one in eight No voters also say they will back Nicola Sturgeon’s party.

True, even many SNP voters are doubtful about the idea of holding another independence referendum as quickly as Ms Sturgeon has in mind. However, nine in ten are in favour of holding one at some point in the future. For most SNP supporters the question is an issue of timing rather than one of principle  and seems unlikely to motivate them to change their minds.

But if the SNP are able to win over much more than a niche market, nobody else is able to do so. For a start, not only does the SNP’s popularity among Remain voters leave little more than crumbs left for their fellow supporters of a second EU referendum - the Liberal Democrats, Change UK, and the Greens - but it also means that, in contrast to the position south of the border, support for Labour among Remain voters (standing at around one in five) is only modestly higher than it is among Leave supporters (one in eight). This does not make it any easier for the party to mount a performance that might mark a recovery in the party’s fortunes.

Even so, there would still seem to be plenty of room for a party that stands for both Brexit and the Union.

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But while that might be how the Conservatives think of themselves, their failure to deliver the former means the formula that served the party so well in 2017 no longer does so now. 

The initial Euro-polling suggests that only just over in five (22%) of those who voted Leave will back the party on May 23. Even many a Leaver voter who would still be loyal to the party in a Holyrood or Westminster election indicates that they will not back it for the Euros.

They are minded instead to protest against the UK government’s failure to deliver Brexit by voting for Nigel Farage’s Brexit Party. It already seems to be the single most popular party among Leave voters, albeit still only backed by no more than one in three, and seems to be in with a good chance of winning one of the six seats that will be up for grabs, matching UKIP’s achievement in 2014.

A Conservative collapse, a weakened Labour party, and the unionist vote scattered to the four winds. What more could Nicola Sturgeon could hope for as she pushes for a second Independence referendum?