IT was an opinion poll lead that collapsed like skittles.

A January snapshot put Labour 16 points in front of an incumbent Nationalist government that was struggling to get on the front foot. Four months later, the SNP has overturned this deficit by recording one of the most astonishing victories in Scottish election history.

If 1979 ushered in Thatcherism, and 1997 marked the beginning of Blairism, 2011 seems likely to go down as the high-water mark of Nationalism.

Not only did the SNP win a majority of seats, but they did so by ending the careers of some of Labour’s highest-profile MSPs.

Andy Kerr. Tom McCabe. Des McNulty. David Whitton. Frank McAveety. All swept aside by the Nationalist tsunami.

“It was ground zero,” said a Labour aide, as his party’s seats tumbled one by one.

Speaking to the Sunday Herald, a collection of Labour candidates, former MSPs and senior party activists have attributed the disaster to the mistakes made by leader Iain Gray since taking over in 2008.

However, many of the party stalwarts said Gray’s woeful campaign was a symptom of the party’s problems, rather than the cause. The roots of Friday’s calamity, said one Labour candidate, go “much deeper” than a six-week campaign.

Ironically, the organisation of Labour’s campaign was far better than in previous elections. In 2007, one senior party figure was horrified to find that a Labour candidate in a winnable seat did not even have a telephone in her office during the campaign.

Four years later, the party had sophisticated software that allowed volunteers to identify potential Labour voters.

Even so, a number of candidates had gripes with the performance of the Labour officials in control of the campaign. This was a team that included general secretary Colin Smyth, spin doctor Rami Okasha, campaign co-ordinator John Park, and aides to Gray.

One source said Okasha, an unelected figure, had a policy role as part of his duties. “This was in spite of him knowing f*** all about policy,” said the insider.

Park also irritated some Labour figures with what was described as a stream of “inane” campaign emails. “Every one of his emails seemed to say it was ‘game on’ for Labour, despite what the polls were saying. It was so vapid,” one said.

A losing candidate was scathing about Team Labour: “If you find a Labour candidate who thinks that any of the senior officials in John Smith House were useful, I’d be surprised. Their inexperience was very apparent.”

However, it was Gray’s strategic mistakes that are being scrutinised, rather than the day-to-day decisions of party officials.

Although Labour’s doorstep strategy was better than it was in 2007, party sources point out that a ground war can only be won if enough voters respond positively to the message being conveyed.

Labour’s twin-track approach – flagging up the Tory bogeyman, and humming an anti-independence tune – failed to catch on.

Crucially, insiders told the Sunday Herald that the SNP landslide reflected the complacency displayed by the party after its one seat loss to the Nationalists four years ago.

“There were two types of MSP after 2007,” a party source said.

“Those who realised the defeat had to result in fundamental change to the party, and those who thought the result was an aberration.”

Senior party sources say a comparison between Wendy Alexander, who succeeded Jack McConnell, and Gray is instructive.

For all her flaws, Alexander won the leadership on a promise of renewal and internal reform.

She called for greater autonomy for Scottish Labour, demanded full control of devolved policy making, and floated US-style primaries for selecting candidates.

The latter was a radical suggestion for ending the hegemony of the local cliques and trade union branches that dominate selections.

Alexander, to the alarm of many of the power-brokers in the party, saw the 2007 defeat as an opportunity to change Scottish Labour from top to bottom.

But Gray, the Labour sources said, ditched the Alexander agenda after taking over and declined to change the party’s unreformed structures, or challenge vested interests.

“Iain was not interested in looking at the privileged role of Scottish MPs or the trades unions in the party, as those were the groups that installed him as leader.”

A senior party source said Gray ignored a review on internal reform commissioned by his predecessor, adding: “I don’t blame Iain for everything, but I blame him for not changing the structures. He said he would reform the party, but he didn’t. It was appalling.”

Gray’s conservatism, another critic said, was visible in almost every aspect of his leadership.

Rather than thinking strategically about campaign messages and policy, the East Lothian MSP fell back on the negative attacks that had failed at the previous election.

In particular, the Labour leader had nothing in his policy locker to reach beyond his core vote and entice the LibDem sympathisers who were switching to the SNP.

“Gray played up the Tory fears and attacked independence because he had nothing else. He failed to come up with one half-decent policy of his own in three years,” said one Labour candidate.

His judgment during the key debates at Holyrood also attracted criticism from Labour MSPs. During the perennial budget wrangle with the SNP, Gray’s shadow cabinet invariably refused to vote for a package that included a council tax freeze.

However, Gray then reversed his opposition to the freeze weeks before the election after fearing his stance would cost votes.

“The council tax policy was symbolic of Iain’s short-termism,” another candidate said. “Instead of having a policy of his own that was rooted in Labour values, developed over time, he opted for a last-minute fudge.”

A third strand of Gray’s failure, another senior Labour insider said, was the way he allowed the party to select woeful candidates.

Whereas the SNP made a point of grooming bright young things like Humza Yousaf and Derek Mackay, Scottish Labour maintained its long tradition of selecting members of whichever clique dominated local constituency parties.

In the Falkirk West marginal, Labour again opted for Dennis Goldie, a former councillor who opposed the repeal of anti-gay legislation in schools.

John Hendry, a veteran councillor and election agent, was the “fresh face” selected in Stirling.

And many of the party list candidates who have stumbled into the Parliament, far from following in the tradition of Donald Dewar, Robin Cook and Gordon Brown, are a motley crue of Parliamentary assistants, relatives of MSPs and nonentities.

“Iain Gray did nothing to reform candidate selection procedures and seek out talented new people,” said a figure on the right of the party. “We are now left with dross.”

Insiders believe it was against this backdrop – a party structure not fit for purpose, an inability to think strategically, and shocking candidates – that Labour fought its most dismal-ever election campaign in Scotland.

As one former MSP said: “Too many not very clever, over-promoted, allegedly senior people thought 2007 was a fluke. It wasn’t. The General Election result in Scotland compounded the complacency. People convinced themselves 2011 would be a shoe-in, with no need to face up to difficult policy questions.”

In this context, Labour’s Holyrood campaign was over before it had even begun.

Despite the shock at the scale of the loss, senior figures were predicting defeat long before the polls closed.

In the penultimate week of the campaign, one candidate told this newspaper that the only way Labour could win was if Salmond “disgraced himself in a public toilet”.

In the week of the poll, and days after Osama Bin Laden was killed, a senior party figure texted: “Do you think they’ll release pictures of Iain Gray’s corpse before the weekend.”

Labour now face a prolonged period of recriminations and soul-searching. This will start with finding a replacement for Gray – no easy task given the diminished stature of the Labour group.

The party will then have to embark on what Gray described on Friday as a “fundamental and radical reappraisal of the structure and direction of Scottish Labour”.

Three years too late, say his critics.