'People want a satisfactory balance between their head and heart'

The urbane, German-speaking ex-journalist has a formidable electoral track record; he is not used to losing. Back in 2007, the then fresh-faced 37-year-old was the SNP's campaign chief and saw his party gain power, only just, with 47 seats at Holyrood. Four years later, Mr Robertson was back in charge and co-ordinated the campaign that saw the Nationalists romp home with, this time, a staggering 69 seats and the hitherto unthinkable majority government.

The First Minister was so pleased by his colleague's performance that Mr Robertson was made, just a few weeks after the startling result, campaign director for "all campaigns". One of his first decisions in the summer of 2011 was to travel to Canada to try to learn the lessons of Quebec's failed attempt at independence in 1995.

Among the many things he learned was to start early, build as big a tent as possible, launch a campaign of unprecedented mobilisation, be positive, raise confidence and seek to instil in people a psychology of change.

Of course, the Moray MP's biggest challenge now lies before him in the six months to September 18. Interestingly, the words he uses most are "positive", "optimistic" and "certain"; the first two perhaps leading to the third.

But why, if there is so much positivity, optimism and certainty, are the pro-independence forces continuing to lag in the opinion polls.

Mr Robertson replies: "Since the launch of the White Paper at the end of 2013, the polling numbers have shown a trend indicating a rise in support for Yes and a decline in support for No, which shows the momentum is heading in the right direction."

I point out that the Yes camp is only improving its position if you strip out the don't knows. I also mention a YouGov poll earlier this month that placed the No vote on 53%, the Yes on 35% and the don't knows on 12%.

The SNP leader at Westminster makes a political side-step and having talked up the significance of recent polls, talks it down, noting: "I humbly remind everybody to remember the polling numbers in 2010/11 and all kinds of assertions being made by a variety of people and what that meant for the Scottish parliamentary elections."

But, of course, as the Quebec experience showed, referendums are not the same as elections.

"We have a little over six months to deliver on a campaign plan to maximise understanding of how Scotland can successfully govern itself economically and socially, that we should do this and that we must do this, so the efforts of the campaign in its different parts is geared to delivering a Yes vote on September 18, 2014, and that's the poll that matters. So every sensible well-calibrated campaign is geared to delivering the right result on the day of the poll that matters.

"Scotland is on a journey and people are actively considering what they should do and all of the evidence I have seen has shown there has been significant movement and only in one direction. Our job is to help those who are as yet undecided and are actively considering whether to vote yes."

The Wimbledon-born politician then makes an even bolder claim. Asked about the scale of the don't knows, he estimates that far from being just one in eight of voters, as the YouGov poll suggested, they are, in fact, more than one in three; in other words, 1.6 million of the four million electorate.

"Up to 40% of the electorate are reachable," Mr Robertson declares. "How one chooses to describe the degree of undecidedness I will leave to the psephologists but there is a very, very sizeable group of voters who are not as yet finally and irrevocably committed to voting Yes or No. Most of them wish to be persuaded of the strongest case for Yes so they can, if they want to, vote that way."

Asked if he really believes the vast majority of the 1.6 million non-committed voters want to be persuaded towards independence, he replies coolly: "Yes. People want to get a satisfactory balance between their head and their heart when they are undecided. Often people have issues they want to work through, want knowledge about certain things they feel they don't have and often, when they do, it helps them firm up their view that voting Yes is the best thing for them, their family, their community and their country."

No doubt Mr Robertson's opponents would regard such optimism and confidence that, say, one million Scots are simply waiting for the Yes campaign to lighten their darkness on independence, as delusional.

"I am totally convinced the referendum is there to be won. It comes down to confidence, optimism and doing everything that we need to do to provide voters with the information that they require to vote Yes."

For the leader of a grouping of MPs that numbers just six in a parliament of 650 members, optimism and confidence are prerequisites for survival.

He quotes none other than the great communicator himself, Bill Clinton, who noted how, regardless of the result, the referendum could be "the making of Scotland".

As Mr Robertson again uses the words "positive" and "optimistic" in relation to the Yes campaign, I ask if the so-called Cybernats enhance or diminish the debate.

He says: "Anybody who uses extreme language or is disrespectful from any perspective diminishes the democratic process.

"There are so many people who have reconsidered how they are going to vote, we need to approach every conversation with every single voter as somebody who is potentially persuadable and one persuades people by being respectful and explaining this better future for Scotland is for everybody regardless of where their views were. The point about the discourse we choose and the way in which we choose to make our case is really important."

One of the key issues that has dominated the campaign has been the issue of the Scottish Government's preferred option of a currency. Despite all three main Westminster parties insisting the idea is "dead", the SNP leadership carries on regardless. But could the Nationalists propose between now and the autumn a Plan B?

Mr Robertson fires off references to the White Paper, that Messrs Osborne, Alexander and Balls are saying what they are saying for political expediency rather than economic common sense and that in the end they will come round because they won't want to "cut off their nose to spite their face". But it is not a no. I ask again.

"Unless you campaign for something, you certainly won't get it," he replies, stressing: "I'm confident that not only will we win a Yes vote but because of the common sense that will prevail, even in Westminster, we will have a shared currency to everybody's benefit."

But having so forcefully rejected a currency union, it would be nigh on impossible for all three main Westminster party leaders, ahead of a May 2015 General Election, to make the mother of all U-turns.

The campaign strategist laughs and points to the Tories' attitude on devolution and the Liberal Democrats' on tuition fees. "In politics, one has to deal with political realities and everybody has committed to respecting the result of the referendum and consequences flow from that."

The prospect of settling the divorce between Scotland and the rest of the UK in 18 months has struck many as fanciful. But Mr Salmond and his colleagues got an unexpected help in this regard when the UK Government's own legal advisor, Professor James Crawford, last year admitted such a timescale seemed "realistic".

"I'm confident we will get there," says the Nationalist. "No country in the world has more experience of trying to go through this sort of transition than Whitehall. In every single case, they have resisted it until the decision has fallen. When the decision falls, it's in everybody's interests to work together and we will manage it well."

Is he suggesting Scotland is the last vestige of the British Empire?

"We live in 2014 rather than in the nineteenth century. There is no direct comparison with the age of Empire. It's part of the evolutionary process of governments from Westminster and Whitehall.

"We are looking for a new form of governance on these islands that will see decision making move from Westminster to Edinburgh but at the same time seek a new settlement with our friends on these islands in England, Wales, Northern Ireland and the Irish Republic, where we can retain the social union and the other benefits we share while having all the advantages of making decisions closer to home."

But what happens if the London/Edinburgh talks get bogged down and there is no deal by March 2016? Is independence postponed?

"Deadlines are quite helpful in negotiations because it helps concentrate the mind. If sensible people make recommendations and goodwill exists and self-interest is recognised, we will work through that time period and come to an agreement to everybody's satisfaction."

Sensibleness and goodwill might, however, be in short supply in fraught negotiations, nowhere more so perhaps than in the area of defence, given the UK spends £34bn a year on the policy while the SNP's independence plans amount to just £2.5bn a year. How can the latter make Scotland safer than the former?

"At the present time, significantly less is spent on defence in Scotland than Scottish taxpayers spend towards UK defence," Mr Robertson points out. "The UK chooses priorities such as Trident nuclear submarines, which do not deter from instability in other parts of the world, while underfunding other parts of the armed forces that are necessary and cut capabilities that are essential, such as maritime patrol aircraft."

The party's defence and foreign affairs spokesman likens an independent Scotland, in defence terms, to Denmark, which he visited to help draw up his party's defence prospectus. £2.5bn is the average annual defence spend of similar sized European neighbours and "will deliver the appropriate capability in defence infrastructure for Scotland, to our benefit and to our neighbours' benefit in the rest of the UK because we will take the northern dimension seriously in a way the UK does not".

Mr Robertson insists any post-Yes vote negotiations on defence would not be a zero-sum game where Scotland wins and England loses or vice versa. "We will be, in some respects, greater than the sum of our parts." A familiar line often used by Unionists to defend the UK.

One area of SNP defence policy that people outwith and even within the party find hard to fathom is the leadership's desire for an independent Scotland to "remain" a member of Nato. How, I ask, can a party vehemently opposed to nuclear weapons want its country to be part of a nuclear alliance.

Mr Robertson, wearing a parliamentary armed forces tie, does not answer the point directly. He talks about accepting the "rule book" of an organisation when one joins it, that, with similar-minded allies, an independent Scotland would seek not to host nuclear weapons - the plan is to get rid of Trident by 2021 - but to "see the concentration of our efforts on the appropriate conventional defence arrangements in northern Europe".

On the other part of his portfolio, foreign affairs, I ask how an independent Scotland of five million people could hope to have the international clout of the world player that is the United Kingdom with all its networks and alliances. Won't it end up as a diplomatic backwater as opponents claim? Mr Robertson disagrees, saying: "A visible Scottish presence around the world would be a huge addition to Scotland's economic potential, to our cultural footprint and to the specific contributions we can make in the international world."

As for the 44-year-old's own ambitions, does he hanker after being first Foreign Secretary of an independent Scotland? He says: "My focus is on the referendum on September 18 2014. Without delivering a Yes result, it's not possible to imagine the most important priorities for Scotland or for me, which are a more socially just society, a more successful and vibrant economy and the ability to have direct relations in a European and international context. Those are my key priorities."

The next six months are likely to be the longest of Mr Robertson's political life.