Normally calm politician is enraged by tone of Scottish campaign which has seen him face regular taunts of being a traitor

With his calm air of authority at the height of the credit crunch, Alistair Darling is remembered across Britain as a steady, if slightly bland, figure.

But the spiky figure of old, who started his political career as a supporter of the Trotskyite International Marxist Group, is making a reappearance as he confronts his supremely confident opponent in the Scottish referendum campaign.

"Alex Salmond is running a week-long Sheffield rally," the former chancellor says of the first minister's helicopter tour of Scotland, referring to the famous moment of misplaced hubris by Neil Kinnock in the 1992 general election campaign.

The man who kept his cool during the financial crash is enraged by the tone of the Scottish referendum campaign, which has seen him face almost daily taunts from independence supporters that he is a traitor.

As the two and a half year referendum campaign hurtles towards a bitter and ugly end, the head of the pro-UK Better Together campaign says: "I have been involved in political campaigns for the last 35 years and have never seen anything like this. There have been dark aspects to this which need to have a light shone on them because they are not acceptable."

In a Guardian interview Darling says that Salmond, the Scottish first minister, needs to think carefully about his role in fomenting a volatile atmosphere. Police were forced to intervene in Glasgow on Thursday when pro-independence campaigners disrupted a welcome party led by the former chancellor for the arrival of a trainload of Labour MPs from London.

"When [Salmond] describes himself as being Team Scotland the implication is that if you are not on his side you are somehow not Scottish. We know exactly what he is doing here – it is the old dog whistle thing. You can be very patriotic and you can vote for the nationalists. You can be equally patriotic and say no thanks."

The Better Together campaign, often lampooned as more in character with the affluent Edinburgh suburb of Morningside than Glasgow's tough Gorbals district, believes that it has conducted itself in a more civilised manner. But the pro-UK side is not afraid to trade the odd low blow.

Salmond is mocked as "el presidente" by the campaign team and Darling makes great play of the first minister's decision to make a US election-style dash around Scotland.

"He is flying around Scotland in a helicopter today but it is the people who are down on the ground who are going to face the consequences of this," the former chancellor says of the first minister's dismissive response to the decision of Scottish banks to move their legal headquarters to London in the event of a yes vote.

The personal digs at Salmond highlight one of the main criticisms of the cross-party Better Together campaign. This is that it has been relentlessly negative and has concentrated solely on shelling SNP lines rather than following the traditional election rubric of outlining a positive and uplifting vision.

Better Together strategists say their plan was deliberately drawn up to highlight what is described as the "absurd monstrosity" of the SNP offer on independence. Private polls showed there was no point in reaching out to two committed blocks on either side – nationalist and unionist – who each accounted for around a third of the vote. The polls indicated that the campaign should instead concentrate on the third of the voters in the middle who are instinctively attracted to independence but who would be put off by any suggestion of a threat to the economy, to their mortgages and to their pensions.

This explains why Darling persuaded George Osborne to rule out Salmond's plan for a currency union between an independent Scotland and the rest of the UK. It also sheds light on why Downing Street is encouraging supermarkets to warn of price rises after a yes vote.

Darling is unapologetic about the strategy, dubbed Project Fear. "If anyone asked you to take a major decision in your life – to get married, to live with someone, to take a completely different break in your career – it would be extraordinary if you did not ask questions, especially if you thought there was no way back if it all went wrong. Being negative is asking Alex Salmond a question to which he does not know the answer."

The strategy appeared to serve Darling well up to his first televised debate with Salmond early last month, when he trounced the first minister over the biggest issue of the campaign. The ever confident Salmond looked, for once, like a cornered animal after he struggled to explain his Plan B if the UK carried out its threat to block a currency union after a yes vote.

Then, within the space of a few weeks, the campaign turned after Salmond comprehensively defeated Darling in their second televised debate when the first minister finally addressed the currency union by joking that he had three plan Bs. These included so called "sterlingisation" – using the pound without the agreement of Westminster.

The surprise – and decisive turning point in that televised debate – came when Salmond mocked Darling as a helper for the Tories, who wanted to privatise the NHS in Scotland. This was untrue because, as Darling pointed out, Salmond is the only person with the power to privatise the NHS in Scotland.

But the debate led to despair in the Better Together camp, where strategists reached two conclusions. Their trump card – the currency – had been neutralised by Salmond, who was now "out-Labouring" Darling with what were described as "intuitive" arguments for natural Labour supporters about a Tory-led threat to the NHS.

Salmond's reward came last weekend when a YouGov/Sunday Times poll gave the yes side its first lead, though the no side has inched ahead in the latest Guardian/ICM and YouGov/Times polls.

One strategist said: "We put a lot of faith in the rationality of the risk based arguments and assumed there would be a point where the [Salmond] bluster - the stock form of rebuttal - would be seen through. It hasn't."

Amid evidence in the polls that Labour voters were moving in sizeable numbers to the yes camp, the time had come to place the party's "iconic figure" in Scotland at the forefront of the Better Together campaign. Gordon Brown, who had reframed the debate in the early summer as a battle about what is best for Scotland rather than a cross-border skirmish, stormed into action this week by announcing accelerated plans for what he described as Scottish home rule within the UK.

The former prime minister is now touring Labour heartlands of the west of Scotland with a series of vintage performances in which he speaks from the heart about the political and familial links across the UK before "shelling" the SNP, as Darling says, with statistics.

Darling, who had a troubled relationship with Brown when they finally achieved the two top jobs, expresses delight.

"Gordon does speak from the heart and when it comes to arguing with his opponents there is no one who knows how to fire the shells better," he says. "We have rediscovered a lot of the energy, zeal and fervour we have always had."

Leading Better Together figures from the other parties believe that the Brown intervention on home rule finally provided – if a little late in the day – the positive message that had eluded the campaign.

On the other side of Edinburgh – at a Liberal Democrat rally at the city's Napier University business school – the magnitude of Brown's intervention was illustrated by Lord Steel of Aikwood, the grand old man of Liberal Scotland.

The former Liberal leader and first presiding officer of the Scottish parliament told the rally: "As we near the end of the referendum campaign I reflect that the separatists have had a good deal of fun and, to be frank, it has not all been entirely wasted. It has flushed out from the Conservative and Labour parties a bolder approach to home rule for Scotland than I ever thought they would accept."

The atmosphere is changing and the Better Together leadership is sniffing victory. The ever cautious Darling is not leaping about just yet but he feels sufficiently relaxed to hint that there may yet be another chapter in his political career after the 18 September poll, possibly in the Scottish parliament.

But he insists his wife will have the final say. "Maggie's got 52% of the vote," says a chuckling Darling.