Well wrapped against the morning squalls blowing in off the North Sea, a group of us huddle outside a barn in the flatlands of rural East Lothian, waiting for Nicola Sturgeon. There’s barely a local voter in sight, but a yellow ribbon has been tied across the barn entrance for Scotland’s first minister to cut during her visit to the Thistly Cross cider makers outside Dunbar.

Right on time, Sturgeon pulls up in a black people carrier emblazoned with two yellow slogans. “I’m with Nicola”, says one. “Nicola Sturgeon for First Minister”, says the other. She says nice things, cuts the ribbon, sips a glass of elderflower cider with co-founder Peter Stuart and does whatever the photographers ask her to do. Then she is off again, heading south into the Lammermuir hills to campaign in the Borders.

East Lothian is one of the Labour seats the SNP hopes to add to its already commanding total in the elections for the devolved Holyrood parliament next Thursday. Sturgeon says she’s here to highlight her party’s plans to boost rural jobs. She takes issue when she is asked whether the SNP’s election campaign is presidential. But the slogans on her car tell the real truth. So does the SNP’s manifesto, whose cover consists of a picture of Nicola Sturgeon and the word: Re-elect.

The SNP is now, incredibly, Scotland’s establishment party. Sturgeon has support from her party activists that David Cameron or Jeremy Corbyn would die for. She is admired by the Scottish public to a degree that no other political leader anywhere in these islands can match. When the SNP again wins the Holyrood elections next week, probably with record support, she will enjoy power and an electoral mandate that few political leaders ever get close to.

Yet what is the purpose of the SNP’s ascendancy? Nine years ago, when Alex Salmond led the party into government for the first time, as a minority, the purpose was to prove that the nationalists were an effective alternative to Labour. That was accomplished with style. Five years ago, when the SNP was returned with an overall majority, the purpose became to reach the goal for which the party existed, independence. That too was transformative, though not quite in the way nationalists expected.

In 2016, however, the SNP’s purpose is rather less clear-cut, even if the mandate this time looks sets to be the biggest yet. Different observers put the party’s current purpose in different ways. To regroup following 2014 and lead the country towards a second referendum, say many. To protect Scots from the Tory government in London and from austerity budgeting, say others. To me, though, it increasingly begins to look as though the prime purpose of the SNP may simply be to keep its all-conquering show on the road.

There’s nothing wrong in principle with a party that wants to win elections. Other parties one could name should try it occasionally. The SNP is certainly brilliant at doing it. The party has the standing, the trust, the record, the knowhow and the machine to deliver big time for Sturgeon next week. There’s no chance of an upset.

Nevertheless, the next five years do not look easy for Sturgeon. The SNP has defied gravity in the past. Yet underlying realities about modern politics and economics apply in Scotland, too. That’s true even when the advantages of office, the decline of the other Scottish parties, and a big majority at Holyrood are taken into account.

If the UK votes to remain in the European Union in June, for instance, then the chances of another independence referendum look slim to vanishing. Even if the UK votes to leave, against Scotland’s will, a second vote is far from certain.

Sturgeon has equivocated throughout the campaign about this, presumably to try to keep everyone on board. It makes activists who were drawn into the party by the excitement of 2014 impatient. At the very least there will be pressure for a much more explicit commitment in 2020 and 2021.

The reason why Sturgeon is uncertain about a second vote will not go away, either. There is not enough support. The big question that the yes campaign failed to answer in 2014 – the currency of an independent Scotland – is still unanswered. Meanwhile Scotland’s oil industry is contracting, both because of extraction costs and because of the fall in the world price of oil.

All this means that Sturgeon faces a fiscal dilemma. To provide all the public goods the SNP wants to provide – which include the free prescriptions, free university tuition and free elderly care on which the party initially built its popularity and progressive reputation – the SNP is increasingly squeezing Scottish local government and education. Yet the party has also refused to raise taxes to pay for them – and indeed intends to lower some business taxes.

Scotland is in many ways a more conservative country than it lets on. So the SNP may have got the politics of this right. Alternatively, the apparent failure of Labour’s efforts in this campaign to make higher taxes a vote-winner may say more about Scottish Labour’s unpopularity than about tax policy.

But the issue is not going to go away. The imminent devolution of even more tax powers to Scotland makes this a much greater public dilemma for Sturgeon than before. It will be harder than before to blame the English.

Over the past decade, the SNP has been swept into power in Scotland as a new political force. What it replaced was in many ways pretty rotten. The question today is how long an SNP that was once the embodiment of new Scottish politics can withstand the pressures that are already transforming it into another establishment. Re-elect Nicola will work in 2016. But in 2021?

Political evidence suggests the answer depends at least as much on its opponents as on the SNP. If the opposition parties wait for public opinion to return to them, the current polls suggest the wait could be a long one. But if the Scottish opposition comes together and reinvents itself root and branch, anything might happen.

A party with Labour’s policies led by a politician as authentic as the Tories’ Ruth Davidson would get a lot of votes. And that is a truth that is not confined to Scotland.