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Halfway through his interview with May2015, Angus Robertson, the SNP’s Westminster leader, declared: “We have to be among the most successful political parties in Europe”.

It’s a difficult statement to challenge. The SNP currently have 6 MPs. In May they are set to win at least 35, and, according to some forecasts (including ours), could win more than 50. Scotland only has 59 seats. They have also run the Scottish Parliament for the past eight years, and nearly won an independence referendum few pundits expected to be close.

Ever since that referendum, the SNP have been gaining in the polls, as we detailed in mid-October. The SNP surge is an extremely recent phenomenon. In early 2014 the party was slightly more popular than in 2010, when it won 20 per cent of the Westminster vote, but it was still a distant second to Labour in Scotland.

That changed as referendum day approached. At the same time as the referendum race tightened, the SNP started to catch Labour in the polls. On 18 September – polling day – YouGov’s daily polls suggested that the SNP were now as popular as Labour, when voters were asked who they would vote for in a Westminster election.

Robertson is succinct. “Something broke on 18 September. Something broke for the Labour Party.”

Since then, the SNP surge has continued unabated. They now lead Labour by at least ten, and sometimes twenty, points in Scotland-wide polls. Last week, Ashcroft’s constituency polls reiterated this: Labour are heading for collapse.

Robertson’s summary is succinct. “Something broke on 18 September. Something broke for the Labour Party.”

This week we caught up with him to ask three questions: Will the SNP soon push for a second referendum? How many seats does he expect they will win in May? And will the SNP form a coalition?

A second referendum

The narrowness of the September referendum has made a second plausible.

Did Robertson expect the race to tighten so much? “Yes.” Why? “Because the case is an incredibly strong case.” The nationalists still lost, but Robertson was encouraged. “As people became better informed, as people thought about things more, you saw a rise in support for Yes.”

So if the referendum had been one month later, would ‘Yes’ have won? “I think so, yeah.”

If the referendum had been one month later, would ‘Yes’ have won? “I think so, yeah.”

But what of a second referendum? Robertson thinks SNP leader Nicola Sturgeon has been “crystal clear” on this, but we entered his Portculis House office unsure.

Do you envision a situation in the next parliament in which the SNP will call for a second referendum? “Well there will only be another referendum in Scotland when people want there to be another referendum.”

But how do we gauge when the people want another?

“Well, so… the referendum in 2014 showed that a great many people wanted independence, wanted more powers for Scotland – not losing sight of the fact that the ‘No’ campaign fought desperately to…”

He drifts onto the shortfalls of ‘the Labour Party’ (never Labour). Eventually he returns to his point. The demand for another referendum “will come from the people”. But does that mean it will come from an election, opinion polls or a referendum on a referendum? Robertson’s answer doesn’t deviate from a script.

“There will only be another referendum in Scotland when people want there to be another.”

“The election in 2015 is an election for Westminster, and on what we can secure in terms of the more powers that were promised. The 2016 election [to the Scottish Parliament] is about re-electing the SNP with First Minister Nicola Sturgeon and her agenda – with the devolved powers of the Scottish Parliament.”

So will the SNP use either or both to suggest Scotland wants another referendum? “I was just stressing that 2016 will be an election for the Scottish Parliament,” is Robertson’s curt reply.

“By then we will know what is being delivered at Westminster when it comes to more powers. And it will become clear about the direction of travel that Scotland will be taking within the constraints of the UK.

“And as that debate goes forward, on the basis of what further devolution is being delivered, people will make a considered view about when we want to take the next steps toward home rule and self-government.”

If it won’t “become clear” whether powers have been devolved until May 2016, does that mean calls for a second referendum made before then would be pre-emptive? And doesn’t this mean the SNP would actually decline any offer of a second referendum before Westminster has the chance to devolve powers?

To preserve the union, Westminster has to deliver the “near federal” powers it promised.

Robertson doesn’t want to confirm hypotheticals. “I can say it ten times round.” “The issue of referenda is for further down the line. What happens beyond 2015 will be determined by what is delivered, and whether people feel they have been short-changed.” “…as they so often have been,” he adds.

His answers may be indirect but the implication is clear. “The referendum came about because the SNP was elected [to the Scottish Parliament] on a mandate of a referendum in 2011.” If powers aren’t delivered by 2016 and the SNP are returned to power in Holyrood, the SNP can claim the Scottish people want a second referendum.

To preserve the union, Westminster has to deliver the “near federal” powers it promised during the campaign.

How many MPs could the SNP win?

As campaign director for the SNP’s 2015 campaign, few people are better placed than Robertson to answer the election’s most uncertain question: how many seats will the SNP take from Labour?

He ran the party’s 2007 and 2011 campaigns for the Scottish Parliament, and they are his guide in 2015. In 2007 the SNP won 32 per cent and Labour 31 per cent. In 2011 they won 45 while Labour managed just 29.

That 2011 result – SNP 45, Labour 29 – is almost identical to Scottish opinion polls since the referendum. An average of the thirteen released so far puts the SNP on 45 and Labour on 27.

The SNP has always performed worse in the Westminster election succeeding a Scottish Parliament election, but Robertson thinks “we’re seeing an end to that Westminster gap”.

Ashcroft’s constituency polls certainly suggested so last week. They implied the SNP will win 48 per cent in May, with Labour as low as 23 per cent.

One election watcher recently suggested to May2015 that when pundits see the SNP’s poll numbers they conclude, ‘Seems high, let’s divide by 2’. A SNP source recently suggested to the New Statesman that they would win no more than 17 Labour seats.

Robertson offers no such suggestion. The party are set to win twice as many Labour seats, and he doesn’t deny he’s hopeful.

“The higher end of the opinion polls would mirror or even surpass 2011. There is literally not a single seat that we shouldn’t be taking seriously. We are working on the basis we can win across Scotland.”

He thinks the SNP will do better than they ever have in a Westminster election. In 1974 they won 33 per cent and 11 seats. “I think we will do better than that in both votes and seats.”

His confidence isn’t just about poll numbers. Like all political parties, the SNP have data on voters across Scotland, and have been gathering it for years. When volunteers knock on doors they effectively conduct polls that are fed back to party HQ. The party can then see how the opinions of erstwhile Labour voters, or those who don’t usually vote, have changed since past elections.

Robertson thinks the SNP will do better than they ever have in a Westminster election.

“It’s not just what’s going on in opinion polls or party membership, it’s actually what’s going on in the ground. Have a look at what’s happening on the high streets of Scotland’s cities, town and villages.”

It’s hard to see how Labour can turn this tanker around in the next three months. The SNP now has nearly 100,000 members. Scottish Labour is estimated to have fewer than 20,000, and a recent leak suggested the party are contacting voters at a pitiful rate.

Polls are only a snapshot. And by May they could worsen for Labour, rather than improve. Robertson has reasons to be confident:

“The most persuasive way to reach voters is face-to-face, it’s not buying billboards, or party political broadcasts, or direct mail. That’s what the UK political parties do, because it’s the only way they can reach people. They don’t have the manpower.”

Coalition or ‘confidence and supply’?

The post-election government has never been more uncertain. The most likely outcome according to the bookies is a Tory-Lib Dem coalition, and even that is only considered 20 per cent likely. (The bookies should be treated lightly: they say a Tory majority is plausible, and nothing suggests that will happen.)

But you can make money betting on this stuff. And Robertson is fairly adamant. The bookies say a Labour-SNP coalition is more likely than a Labour majority, Lab-Lib coalition or Tory-Ukip coalition – but it is far from his mind.

“History teaches you that minor coalition partners tend to suffer the brunt at election time”.

“The most likely outcome that we are looking for and preparing for is confidence & supply. Frankly, it is the only one I am spending any time thinking about.”

“History teaches you”, he suggests, “that minor coalition partners tend to suffer the brunt at election time”.

He is unequivocal. “The Lib Dems threw away any sense of bargaining strength”. They were dragged into taking unpopular and bad decisions when they could have exercised more influence. They “deserve the wrath of the electorate”.

His opposition is two-fold. One is general: coalitions prop up major parties and punish smaller ones. The other is specific to the UK.

The way coalitions are formed in the UK “cannot possibly withstand the pressure of events”. The speed at which the Tory-Lib deal was struck in 2010 was almost historical comparison.

“Frankly, confidence and supply is the one outcome I am spending any time thinking about.”

“Look at how long it takes the Germans – and in what detail. Given that is not the experience of politics in the UK…I think there are a lot of advantages to confidence and supply.

“Because no party is likely to have the detail of its full legislative programme worked out, it makes sense to be able to reserve your position on different proposals until you actually see the detail of what those positions are.

“There are lots of examples – not least in Scotland, but in other countries too – where that’s worked.”

If Ed Miliband can’t form a coalition with the SNP, he will need to unseat David Cameron without a majority. If so, he will have to pass a Queen’s Speech on a confidence and supply deal.

If the SNP loses confidence in that deal, he could be in Number Ten but without the majority needed to govern. That is a precarious proposition on which to unseat a sitting Prime Minister.