GORDON Brown's rally at the Old Fruitmarket in Glasgow last Tuesday night was quite an event.

More than 900 people turned up to hear the former Prime Minister and remind themselves what a good, old-fashioned tub-thumping orator he is. They also listened to passionate contributions from Scottish Labour leader Johann Lamont and council chief Gordon Matheson. The crowd was even treated to a song by performer and Labour activist Katie Murphy.

The mood was upbeat (though there was a sharp collective intake of breath when Mr Matheson accidentally introduced Katie as Karie. That would have been a strange booking). At the end of it all folk drifted off into the Merchant City apparently happy with their evening and, possibly, a tiny bit inspired.

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Two things were striking, the first that this was happening at all. The Yes campaign has been holding meetings up and down the country for months, packing town halls and community centres night after night with supporters and as many undecided voters as they can find. Either collectively as Better Together or in their own party campaigns (Mr Brown was speaking on a United With Labour platform) the main pro-UK parties have been much slower into the fray. The second surprise was the backdrop: two vast banners in eye-popping red and yellow bearing a single word: No.

The political weather is changing as the two campaigns prepare to mark 100 days to the referendum on Monday. For Better Together and especially Labour that means embracing No, a word previously eschewed for fear of sounding negative. No is now good. It's positive even, if that's possible. We're No and we're proud, Labour luminaries from Mr Brown to John Reid have been telling us all week. It's the cue for Unionist lapels everywhere to be stripped of mealy-mouthed "Let's stick together" badges and re-adorned with bold new "No thanks" buttons.

There is a good reason for this and the clues are in the language that's creeping into Labour speeches. "We hope to give you confidence in our ideas and arguments," Ms Lamont said at the start of her address on Tuesday. She said much the same a couple of days later when she welcomed Barack Obama's backing for a "strong, robust, united and effective" UK. "For the many Scots who have decided or are even leaning towards a No vote, the endorsement of such a powerful and iconic figure will give them real confidence in their argument," she commented.

The approach is aimed squarely at a key group in the referendum fight: those traditional Labour supporters who, to a greater or lesser extent, are swithering over which way to vote on September 18. Some are genuine don't knows, others are "soft Nos," open to persuasion in the final, frantic weeks of the campaign. Labour strategists believe most will plump for No because that's what their heads are telling them even if there's an emotional tug towards Yes in their hearts. Their hope is to shore up support for No and, importantly, to make people feel good about their choice. The plan is not just about winning the referendum. Labour's big fear is that if people vote No without any great conviction they will endure a bout of buyer's remorse and back Alex Salmond to return as First Minister in the 2016 Holyrood election. A No vote, then, will be presented as a positive, patriotic way to protect all that's good about Scotland not just today but for future generations.

The tone is changing within the Yes campaign too, hardly surprising since it is targeting the same group of old Labour heartland undecideds. Having weathered a gruelling few months during which the economic prospects of an independent Scotland have been picked apart not just by the pro-UK parties but by business leaders and academics, the Yes campaign plans to move on from dry arguments about numbers and instead present an inspiring vision of what's possible. "Let's do this - that will be part of the message," a senior Yes Scotland figure told me this week. "We've made the case we can do this. Now we need to create a sense of urgency, a sense of history, a sense of opportunity and say to people, let's seize the day, let's do this."

With final plans being laid, the scene is set for a dramatic last 100 days.