The 1959 general election was the first time the Conservative party failed to win the most seats in Scotland. The Tories have been in decline in this part of the UK ever since. The lowest possible point was reached in 1997, when they were wiped out in Scotland completely.

Four years later the Tories were back, thanks to the solitary gain by Peter Duncan of Galloway & Upper Nithsdale. The party won the constituency from the SNP: a feat that seems remarkable when placed against the political landscape of today.

When it was announced that Galloway & Upper Nithsdale was to be abolished at the 2005 election, it seemed as if the Tories were set to be wiped out in Scotland once more. But come that election David Mundell achieved a surprise win in the newly-created constituency of Dumfriesshire, Clydesdale & Tweeddale, a seat that had been 96th on the Tories’ target list. Mundell duly replaced Duncan as the Conservatives’ only Scottish MP in the House of Commons, a distinction he has held ever since.

But could that be about to come to an end?

Up until last autumn, the main threat to Mundell was thought to be from Labour, who need a swing of 4.6% to take the seat.

Now a more plausible challenger might be the SNP. Admittedly the nationalists would need a mighty swing of 13.6% to win. But last week’s polls by Lord Ashcroft suggested the SNP were already achieving those kinds of swings – and much higher ones – in some of Labour’s safest seats. Might they manage the same thing against Mundell?

It would foolish to extrapolate any sort of pattern from Ashcroft’s polls and map it right across Scotland, including on to the nation’s only Conservative seat. It’s also important to consider the particular demographic and voting habits of Dumfriesshire, Clydesdale & Tweeddale.

Within the constituency boundaries lie two of the three council areas that voted No most strongly in the independence referendum: Scottish Borders (66.56% No) and Dumfries & Galloway (65.67% No). Only Orkney (67.20%) registered a higher No vote than these two regions.

The constituency also contains part of the South Lanarkshire council region, which voted 54.67% No. South Lanarkshire council is currently ruled by a Labour majority, who have 34 of its 67 seats. The SNP is close behind, however, on 26.

By contrast, Labour has no seats at all on Scottish Borders council, where the Tories are the biggest party, but which is run by a coalition of the SNP, independents and the Liberal Democrats. Dumfries & Galloway council, meanwhile, used to be run by a Tory-SNP coalition (yes, it does happen!) until seven Tory councillors resigned and a Labour minority took control.

A very mixed picture, then. Even the three areas contained with the constituency’s name have different histories of voting. Dumfriesshire was traditionally the Tory heartland, Clydesdale performed the same function for Labour, while Tweeddale had been part of Lib Dem-supporting constituencies since the 1980s – most famously as part of David Steel’s seat of Tweeddale, Ettrick & Lauderdale.

All of this makes it very hard to predict what’s likely to happen on 7 May. If traditional Labour and Lib Dem supporters switch to the SNP, it might be enough to see the SNP’s candidate Emma Harper squeeze past Mundell into first place. But the Scottish Green party is also standing, which may splinter the anti-Tory vote and allow Mundell to cling on. And if Ukip decide to stand, as they did in 2010, things may become even more unpredictable.

My instinct at present is that Mundell will survive. Voters will fail to coalesce around a single anti-Tory candidate and in the process enable him to slip through the middle and win a third term as MP. But it will be a close result – closer than in 2010, when Mundell had a majority of 4,194.

The Conservatives may not be wiped out in Scotland at this election, but they may face just such a fate come 2020.