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Up until last autumn, the Liberal Democrats had been gradually reconciling themselves to their impending fate: losing up to 20 seats in 2015.

But, like Labour, they thought their Scottish MPs were relatively safe. They perceived the main threat to be in England – from Labour and the Tories. They even contemplated making Jo Swinson, MP for the marginal Scottish seat of Dunbartonshire East, the new secretary of state for Scotland.

The Scottish referendum, and the opinion polls that followed, changed all that. The Lib Dem’s standing in Scotland is no better than in England – the polls suggest they have lost two-thirds of their voters both south and north of the border. As with Labour, a near-wipeout of the Lib Dems is predicted. But, as with Labour, the reality is likely to be more nuanced.

Of the Lib Dems’ 11 Scottish MPs, at least three are almost certain to survive. Here’s why.

When I looked at the Labour-SNP battleground, I argued there are no easy seats for the SNP to win.

When I recently looked at the Labour-SNP battleground on May2015, I argued there are no easy seats for the SNP to win. The same applies for the Lib Dem-SNP battleground. If anything, the challenge is steeper. The SNP needs a swing of 6 per cent – i.e. they need to overturn a majority of 12 per cent – to win its first seat from Labour (Ochil & South Perthshire).

It must do even better and achieve a swing of 7 per cent to make its first gain from the Lib Dems. But if it succeeds, two Lib Dems are at risk: Alan Reid in Argyll & Bute and Christine Jardine in Gordon.

These have to be considered the two most likely Lib Dem seats to fall to the SNP – particularly as Gordon is being contested by the former first minister Alex Salmond, who’s bound to attract not just a huge pack of reporters but a similarly-sized retinue of SNP supporters.

But for the SNP to make any more gains from the Lib Dems, it will need to push the swing right up to 11 per cent (overturning majorities of more than 20 per cent).

Of the Lib Dems’ 11 MPs, at least three are almost certain to survive.

The next Lib Dem seat at risk will be among the election’s most watched: Inverness, Nairn, Badenoch & Strathspey, constituency of Danny Alexander, George Osborne’s effective deputy at the Treasury.

Whoever gets to fight this seat for the SNP has the chance to create the sensation of election night, reminiscent of Tony Benn’s shock defeat in 1983 and Michael Portillo’s in 1997. So far, the party is still yet to pick a candidate.

Here’s the full list of the swings needed by the SNP to defeat all 11 Lib Dem Scottish MPs:

As you can see, only three would fall on a swing of up to 11 per cent. But if the party can nudge the swing even higher, to 12 per cent, another three seats are at risk: Caithness, Sutherland & Easter Ross; Edinburgh West; and West Aberdeenshire & Kincardine.

This would bring the number of SNP gains from the Lib Dems up to six, and Nick Clegg will have lost over half his Scottish MPs.

But for any more Lib Dem seats to fall, the SNP needs a swing of around 15 per cent, at which point Dunbartonshire East and Fife North East – seats in which the Lib Dems beat the SNP by 30 per cent in 2010 – become SNP targets.

Jo Swinson, threatened in Dunbartonshire East, was nearly made secretary of state for Scotland.

Jo Swinson’s seat of Dunbartonshire East is truly a Labour target; the party was only 2,184 votes behind the Lib Dems in 2010. But if you combine Labour’s apparent collapse of support in Scotland with the Lib Dems’ collapse across the UK, you might end up with the SNP benefiting from both parties’ decline.

That just leaves Michael Moore in Berwickshire, Roxburgh & Selkirk; Charles Kennedy in Ross, Skye & Lochaber; and Alistair Carmichael in Orkney & Shetland. I suspect all three are beyond the SNP’s reach. A swing of over 18 per cent would be needed to defeat the first two, while an unbelievable 26 per cent swing is required to unseat Carmichael.

There are two further factors that may limit the SNP’s advance.

One is how poorly the party fared across the Lib Dems’ 11 seats in 2010. In only one of the constituencies – Gordon – did the party finish in second place. In five they came third, and in the other five they came fourth. To win Edinburgh West in 2015, for example, the SNP needs to leapfrog not just the Lib Dems but the Tories and Labour as well.

The SNP won few votes in many of these seats in 2010.

The other is incumbency. So far only two of the 11 Lib Dem MPs have announced they are standing down: Malcolm Bruce in Gordon and Menzies Campbell in Fife North East. The long tenure of MPs like Charles Kennedy (Ross, Skye & Lochaber), Sir Robert Smith (West Aberdeenshire & Kincardine) and John Thurso (Caithness, Sutherland & Easter Ross) are likely to make their defeat even more challenging.

Ashcroft’s polls, and past academic research, have both shown a strong incumbency effect for Lib Dems MPs in England. The party has spent decades building relationships in many of these seats, and will focus all their energy on retaining their MPs.

We can divide the SNP’s Lib Dem targets into three groups:

1. Easy (2)

Argyll & Bute; Gordon.

2. Tricky but not impossible (4)

Inverness, Nairn, Badenoch & Strathspey; Caithness, Sutherland & Easter Ross; West Aberdeenshire & Kincardine; Fife North East.

3. Hardest (5)

Edinburgh West; Dunbartonshire East; Berwickshire, Roxburgh & Selkirk; Ross, Skye & Lochaber; Orkney & Shetland.

My instinct is that the SNP will advance through the first category easily and go some way into the second category, but no further. That would mean 5-6 seats.

The Lib Dems would probably class this as a “good” result. But if the SNP somehow manages to chip away at the third category, Nick Clegg could be left with a party so small that it ends up behind the SNP in the final tally of seats at Westminster. That could mean not only humiliation, but no place for the Lib Dems in a future coalition government.

How many seats could the SNP win overall?

If the SNP manages a swing of 12 per cent against both the Lib Dems and Labour, they would deprive the former of six MPs and the latter of eight. That translates into just over half of the Lib Dems current tally of Scottish MPs, but only one fifth of Labour’s total.

This is obviously at odds with what the latest opinion polls are suggesting – as reflected by May2015’s polling-based prediction model – but illustrates just how much of a struggle it will be for the SNP to capture a majority of Labour seats. For the nationalists to even take half of Labour’s Scottish MPs, they need a swing of 15 per cent. Uniform swings that size are effectively unheard of in general elections.

Historical precedent may hold little meaning in post-referendum Scotland, but an SNP landslide on the scale suggested by the polls would have to rewrite history.

Ian Jones is a contributing editor to May2015. He blogs daily at UKgeneralelection.com.