You know the unionist campaign against Scottish independence may be flagging when its strategists believe that its secret weapon is…  former British prime minister Gordon Brown:

Tavish Scott, the Lib Dem member of the Scottish parliament (MSP) for Shetland, says Mr Darling and Better Together have done well at providing an intellectual case for remaining in the UK, but have failed to connect with crucial sections of the electorate such as traditional Labour voters. Mr Scott wants major Labour figures in Scotland such as former prime minister Gordon Brown and former UK minister John Reid to take a greater role in shoring up “soft Labour” support for the union.

What’s clear is that the ‘Yes, Scotland’ campaign in favor of independence is gaining momentum, while the ‘Better Together’ campaign is losing steam.

A Panelbase poll conducted between April 27 and May 4 shows that the ‘No’ side would win 46% of the vote and the ‘Yes’ side would win 41% of the vote, with 14% undecided. Though Panelbase has typically shown a stronger ‘Yes’ vote than other polls, its findings are consistent with other surveys over the past month. While ‘No’ continues to lead ‘Yes,’ sometimes by double-digit margins, there’s no escaping that the polls are tightening.

That’s causing some alarm within both government and opposition circles. Though British prime minister David Cameron almost certainly believed that most Scottish voters wouldn’t support independence when he agreed to the terms of the referendum with Scottish first minister Alex Salmond last May, his governing Conservative Party must now face the prospect of a too-close-to-call referendum in Scotland just eight months before the wider UK general election in May 2015.

If Scotland votes ‘yes,’ or even comes close to endorsing independence, some senior Tories are already wondering if Cameron will have to resign — after 307 years of union with  England, he’ll be the prime minister who ‘lost’ Scotland.

With the Scottish Labour Party largely leading the charge against independence, what will it say about the generation of national Labour leadership, including includes Scottish-born prime minister Tony Blair, that delivered devolution  Scotland in 1997?

More fundamentally, however, why, so suddenly, does the ‘Yes’ campaign — once deemed hopeless — now seem like it has a chance?

It’s partly because, unlike the esoteric 2011 referendum on replacing the first-past-the-post system with the proposed Alternative Vote favored by the Liberal Democrats, polls over such an emotionally charged topic were almost certain to narrow as the September 19 vote approaches.

It’s partly because there’s an enthusiasm gap between those who want to make an active change in Scotland’s status and those who simply want the status quo. Proponents of independence are more energized to vote and actively campaign.

It’s partly because the European Union supranational structure that guides so much law and policy significantly lowers the risks for an independent Scotland. That assumes that an independent Scotland, which would have to apply for admission to the European Union, would quickly gain to EU membership. There’s some risk that Spanish prime minister Mariano Rajoy, loathe to encourage Catalan separatists, might block Scotland’s admission. But Scotland, having implemented the EU’s acquis communautaire into Scottish law over the past four decades as an EU member, could conceivably accede faster than Austria, Finland and Sweden did in 1995.

It’s partly because Scotland’s first minister, Alex Salmond, the leader of the Scottish National Party, is so much more popular than any figure on the unionist campaign — either within the Conservatives or Liberal Democrats in government or within the opposition Labour Party. The Tories have been massively unpopular in Scotland since the days of prime minister Margaret Thatcher (they won just one out of 59 Scottish constituencies to the House of Commons in the 2010 parliamentary elections). Nick Clegg, deputy prime minister and leader of the Liberal Democrats, the junior partner in the Conservative-led coalition government, is almost as unpopular as Cameron. But Scottish voters, who overwhelming opposed joining the US invasion of Iraq in 2003, soured on Labour as much as everyone else in Great Britain over the course of the last decade.

Mostly, however, it’s because the ‘No’ campaign has waged a much more negative campaign, which is showing signs of causing a backlash within Scotland.

Alistair Darling, the Scottish-born former chancellor of the exchequer under Brown, is leading the cross-party ‘Better Together’ campaign, and he’s been quick to tamp down talk that there’s a momentum shift.

But the latest sniping over whether an independent Scotland could continue to use the British pound is representative of the self-inflicted problem Darling and the ‘Better Together’ team faces. After Tory chancellor George Osborne, and other unionist figures  all tried to rule out a currency union with an independent Scotland, an unnamed government minister told a reporter for The Guardian late last month that ‘of course’ Scotland would retain the pound:

A currency union will eventually be agreed between an independent Scotland and the remainder of the UK to ensure fiscal and economic stability on both sides of the border, according to a government minister at the heart of the pro-union campaign….

“Of course there would be a currency union,” the minister told the Guardian in remarks that will serve as a major boost to the Scottish first minister, Alex Salmond, who accused the UK’s three main political parties of “bluff, bluster and bullying” after they all rejected a currency union. The minister, who would play a central role in the negotiations over the breakup of the UK if there were a yes vote, added: “There would be a highly complex set of negotiations after a yes vote, with many moving pieces. The UK wants to keep Trident nuclear weapons at Faslane and the Scottish government wants a currency union – you can see the outlines of a deal.”

The damage has less to do with the economic merits of a currency union (versus a Scottish currency or joining the eurozone), but the duplicitousness of the ‘No’ camp. Of course Scotland would be able to keep the British pound, at least initially, and there’s little that the ‘United Kingdom of England, Wales and Northern Ireland’ could do to stop it. Mark Carney, the governor of the Bank of England, has essentially admitted as much.

Instead, it’s the iron-fisted tactics of the unionist campaign that are pushing undecided Scottish voters toward the ‘Yes’ camp. Earlier this week, Darling suggested that the rest of the United Kingdom should be able to hold a referendum to determine whether Scotland could enter into a currency union with it. That indicates that Darling and the ‘Better Together’ team still don’t understand why the remarks last month have been so damaging.

In a debate over something as fundamental as independence, which requires such a leap into the unknown, the side that voters trust most will win.

Salmon, the leader of the Scottish National Party, who’s still popular after seven successful years as first minister, has always had a slight edge over Cameron, Darling and other top unionists on the ‘trust’ front. The gap between the public bluster and the private admission from government officials, however, threatens to widen Salmon’s edge.

There’s still time for the unionist camp to turn around its campaign. But to do so, it has to show Scottish voters that they can trust it to present an honest account of the benefits of unionism — and the drawbacks that Scotland would face if it become independent. Otherwise, it risks that voters will dismiss all of its arguments as fear-mongering, delivering Salmond and the independence movement a once unthinkable victory.