IT started as a dark office joke.

But Better Together isn't laughing any more. When the Sunday Herald revealed last month that some members of the pro-Union campaign were privately referring to themselves as "Project Fear", it went viral.

Soon after bloggers and Twitter users picked up on it, SNP headquarters and the Yes Scotland campaign had latched on.

SNP MPs even raised Project Fear in the House of Commons, taunting the Prime Minister and his Scotland Office ministers about it.

"It was a joke in the office," admitted one of those in the No camp, who said it was a riposte to the SNP's knee-jerk claim that everything from the Unionist side is scaremongering.

But was it purely a joke, or was it a more telling Freudian slip?

Now the phrase Project Fear has escaped, there's no going back.Indeed, Yes Scotland intends to build what it sees as a Better Together own-goal into the bricks of the referendum campaign.

Although they aren't officially calling themselves Project Hope yet – that's still at the office joke stage – they are emphasising the contrast between hope and fear in public.

"This was cocky and dangerous from the other side, and it's a big window of opportunity for us," said one of those in the Yes campaign.

"There's been a shift over the last two or three weeks. You can see the news value of all these negative reports from Westminster and Better Together getting less and less. They're suffering from diminishing returns."

The episode has turned a spotlight on the power or otherwise of negative campaigning in the independence debate.

Some involved believe that, just as the Yes side has no choice but to sell hope, the No side has no choice but to sell fear, as simply selling the status quo never satisfies voters.

For Yes Scotland, there's no doubt which wins. Its strategist, Stephen Noon, has a mantra that positive campaigns always beat negative ones. Think of President Obama's "Change you can believe in" from the 2008 US election.

Alex Salmond states it as fact, too. One of the big innovations under Salmond's second-round leadership of the SNP was the use of positive psychology in campaigning, talking up Scotland's "can do" side, rather than moaning about being thwarted by the UK.

The party hierarchy credit this shift, in 2005 and 2006, with helping them to win and hold power.

It seems good evidence, but in elections politicians are essentially selling themselves – "Vote for me, not the other chump" – whereas the referendum is about a separate proposition.

Noon freely admits Labour's "divorce is an expensive business" campaign in 1999 damaged the SNP in the first Holyrood election.

After that, the SNP parked the independence issue by making it subject to a referendum it honestly thought it would never hold.

Asked to back up his claim that a positive campaign always beats a negative, Noon refers to psychological research from the US that found upbeat presidential candidates won 90% of the time against relatively downbeat ones.

But when the Sunday Herald tracked down the original research, it was based on analysing the "pessimistic rumination" exhibited in a single speech from each candidate. Even its authors cautioned against generalising its findings to other elections.

Pressed on the point, it's clear the iron rule that positive campaigning wins is actually an assertion.

"Is this a belief or a fact? It's a belief," says Noon.

Negative campaigning also has a proven, if shabby, record of success.

Exhibit A: the constant attacks on Labour leader Neil Kinnock in 1992 that helped John Major squeak back into Downing Street.

But Noon is undaunted, saying the unionist parties have become stuck in a rut: "They've been running the same campaign for decades, the same things over and over again."

Professor James Mitchell of Strathclyde University, who co-wrote a paper on how the SNP's positive messages helped it win in 2007, said the referendum would be "a classic case of hope versus fear".

He said: "There is no universal law on positive versus negative campaigning. What the SNP showed in 2007 and 2011 was that positive campaigning works if you want to win government. But there is ample evidence that negative campaigning works too.

"We simply don't know the precise conditions under which the mix of positive and negative campaigning works best. Credibility is important. A campaign promising a positive future that lacks credibility is pointless."

At the moment, Better Together has a twin-track "head and heart" pitch for the Union.

The head part is what we've seen most of so far – the analytical papers from Whitehall, the questions over the detail of EU membership, the currency and the welfare state – all of which are legitimate areas for debate, but which are not exactly looking to the sunny uplands.

The second part, the heart, comes next, with emotional appeals to shared history, shared sacrifices and shared institutions like the NHS, and the rest of the UK's gratitude for Scotland's ongoing contribution to the Union.

But if the head argument starts to backfire, can Better Together rely solely on the heart?

"If they go positive, what they're left with is cultural nationalism," says one Yes insider. "It'll be Kate's baby, the Union Jack, the British Lions, all that stuff – a cultural sentiment in favour of Great Britain. But Scots might just find that annoying or underwhelming. The heart is an unpredictable place."

A Better Together source said the Nationalists' obsession with Project Fear was a bid to distract voters from the truth of independence.

"Alongside Salmond talking about Bitter Together in Parliament, Sturgeon taking to the airwaves to feign mock outrage about a phrase that nobody used or uses in anything remotely approaching a serious way, is the most cringeworthy episode of the debate so far."