SHETLAND is unfinished business for the SNP, says Professor Sir John Curtice.

Scotland’s northernmost seat was one of the few areas that resisted the SNP tsunami at the 2015 election, and the Nationalists have craved it ever since.

Scotland’s leading polling expert says this helps explain why Nicola Sturgeon has poured money and manpower into a by-election tomorrow that, on paper, looks an impossible task.

When former LibDem leader Tavish Scott won the seat for a fifth time in 2016, he did so with a whopping 44.3 point lead over the SNP.

Although a good chunk of his personal vote may not transfer with his quitting Holyrood in July to work for Scottish Rugby, what remains is formidable.

For the SNP to win, they would have to not only overturn the safest majority at Holyrood, but end a 70-year-old liberal monopoly in the Northern Isles by converting an awful lot of Unionists to the Yes movement.

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Shetland recorded the fourth biggest No vote in Scotland in 2014.

Nevertheless, Ms Sturgeon has abandoned “expectation management” as political anoraks call caution, and thrown herself into a high-profile push behind SNP underdog Tom Wills.

She has visited the islands three times since July, promoting her party’s odd message that tomorrowThursday is about Boris Johnson, Tories and Brexit as much as Shetlanders. Either an historic upset or an awkward anti-climax awaits.

Her repeated presence has also drawn attention to the fact that in her five years as First Minister, Ms Sturgeon has never visited Shetland except during an election campaign. She was criticised yesterday for only pretending to care about the islands when chasing votes.

“The First Minister is oblivious to the islands when she’s in Edinburgh. Perhaps making the journey to Shetland herself will have made her realise just what a problem expensive and under-resourced transport links are,” said LibDem candidate Beatrice Wishart.

“Shetland’s not just for elections.”

The LibDems, meanwhile, have run an old-school campaign about local services, blaming an indifferent 12-year-old SNP government for all ills.

The cost of ferries has been one of their key themes, forcing Mr Wills to announce that, if he wins, he’ll be “on the phone to Nicola Sturgeon on Friday morning” to tell her what’s what and secure a 20 per cent fare discount.

When a party effectively has to run against itself like that, you know its record is causing problems.

“The Nats have always had their eye on Shetland,” says Prof Curtice. “It’s one of the three places they didn’t get in 2015, and they have been desperate to get their hands on it ever since. So it’s not surprising that they should try.

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“The LibDem candidate is saying, basically, ‘I’m a Shetlander, I’m going to defend your interest, these are the things that the Scottish Government doesn’t do for us up here’. Classic LibDem Celtic fringe rural area stuff.

“She may get 40% of the vote and the SNP 35%. It will be quite a shock if the SNP were to pick it up.”

The SNP have undoubtedly put a shift in, with a third of their MSPs visiting.

The recent European election also showed the numbers tilt dramatically if a third party or candidate does well.

In May’s European poll, the Brexit Party won one fifth of the vote, and the LibDems came 250 ahead of the SNP.

In past elections, Independent candidates have also dented LibDem support in Orkney and Shetland. Although the Brexit Party isn’t standing tomorrow, four Independents are.

Old hands say that, especially with a low turnout, it is Independents who could eat into the LibDem vote and cause Jo Swinson’s party trouble.

An electoral quirk that allows parties to spend £100,000 in byelections regardless of the size of the electorate - and Shetland’s 17,810 is the second smallest in Scotland - has also helped the better-off party flood the area with campaigners, leaflets and social media.

However, LibDem sources insist the party’s vote is holding up.

“It’s still a closely fought race and will likely be much narrower without Tavish’s incumbency,” said one activist.

“But over the past few days our vote has been firming up and undecideds are breaking more heavily for us. Turnout will be key but we’re comfortable we’ll have enough activists to get our vote out.

“It’s clear the SNP leadership have bet the farm on this. This place has been overrun by ministers, but you get the feeling Shetlanders just want their streets back and for this all to be over.

“I’m going to stick with what we’ve got” is a line I keep hearing on the doors.”

Mr Wills said: “Island communities do face real challenges, and I’m ready to take islanders concerns directly to Holyrood.

“Our next MSP will have 18 months to prove their merit before the next Holyrood election. In that time, I believe I can prove what a difference voting SNP can make.“More and more people are preparing to vote SNP on Thursday – many for the very first time.

“It’s true we’re underdogs in this election, but I truly think the tide is turning in Shetland.”

If the SNP do win, Prof Curtice says it won’t “tell you anything about the rest of Scotland” because Shetland has always been one of a kind.

“The place has been utterly sui generis throughout the whole of the post-war period. The Nats are still riding high and Tavish Scott clearly has a personal vote, not all of which will necessarily transfer. But, given the LibDems are back in business generally, it would take something for the SNP to win.”

Scottish Labour candidate, Johan Adamson said: “I have really enjoyed campaigning and look forward to Friday and the results. I believe we have run a very positive and genuine campaign, sharing some bright ideas on tackling the disadvantages Shetland faces in terms of transport and fuel poverty which I hope we can take forward, with the support of Shetland Labour Party and Labour nationally. It has been an exciting election so far and although this is said to be a safe Liberal seat, the decision is far from a certainty.”

A Scottish Greens spokesperson said: “Whatever happens at the polls this by-election has been fantastic for the Greens. It has allowed us to build capacity in Shetland and our local candidate Debra and her team have had a great reception on doorsteps.

“People tell us they are concerned about the climate emergency and we’ve been able to move it up the agenda, challenging the other candidates and government to ensure Shetland benefits from a just transition. We’ll be holding them to account to make sure they deliver that.”