"There will be either a referendum or a constitutional crisis," he said. "Scotland has a clear majority which will ask for that referendum because [the Scottish Green Party] will support it. Therefore that will become the official policy of Scotland. It will then be up to Westminster to say whether it's going to grant or block [the vote]."
On Monday, Sturgeon confirmed that she will ask London to give Edinburgh legal powers to hold a vote under section 30 of the Scotland Act. If Westminster gives the green light to the referendum, it is most likely to take place in late 2018 or early 2019. If it doesn't, "something very substantial" will likely take place, McAlpine noted. He did not rule out possible civil disobedience in Scotland if May decides to block the vote.
In McAlpine's opinion, some of those who voted against Scottish independence might change their mind if London interferes with the democratic process.
"I think it will be very risky for the UK government," he said. "If [London] is serious about the long-term membership of Scotland in the United Kingdom, it needs to fight and win the referendum."
McAlpine described a referendum as an "imperfect vehicle" for resolving big national questions, but added that Scotland has no other alternative. "We are in a situation now where so much has changed, so many substantial things have changed since 2014 that asking the question again five years later does not seem to be anything other than normal democratic politics," he said.
"I am absolutely certain from where we stand right now looking forward that anyone who says that they know the outcome of this referendum is being overly confident. It is perfectly possible for this referendum to go either way," he said. "The independence movement has been holding meetings. We have been preparing for a campaign ahead. We have a good sense about how it is that we can bring wavering voters round to support the case for Scottish independence. And above all I just think that a case for a progressive internationalist outward looking United Kingdom has weakened enormously. So we are fighting with a stronger case against a weaker case. And I do think on balance we will win this one."
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