Until this week, the plan was an undeniable success. But for the first time, that apparently seamless progression has been interrupted. A black mark has appeared on the grid.
Labour's victory in the Glenrothes by-election has halted the SNP's electoral march through Scotland and raised questions about the party's ability to deliver its ultimate goal of Scottish independence.
Critics, inside and outside the party, have started asking, if the Nationalists cannot win over the voters of Glenrothes, what chance do they have of convincing those same people to back the big question of independence?
The Glenrothes result was unexpected and the way the election developed, particularly late on, was extremely unusual.
An hour-and-a-half after the polls closed on Thursday night, a divide appeared between the bright lights of the television cameras on the balcony and the ballot counters, smart and uniform in black polo shirts, flicking through voting papers in the sports hall below.
TV commentators told viewers the SNP was on course to win, but in the background the piles of papers marked for Labour's Lindsay Roy were stacking up.
Word started to spread around the hall that Labour might just win. Rumours of a 1,000 majority circulated, then 2,000, then 5,000, but no-one even suggested such a comprehensive margin as a majority of nearly 7,000.
By the time Mr Roy arrived, shortly after midnight, the SNP had already conceded defeat.
Mr Roy and his wife, Irene, were greeted with raucous cheers from Labour activists while SNP tacticians huddled in a corner over a laptop, some in tears, some just looking grey, tired and defeated.
Their fight was over. That much was clear when Peter Grant, the SNP candidate, arrived soon after Mr Roy to silence. No-one from the Nationalist camp had the energy or the will to cheer him in.
One SNP insider argued that the Nationalists believed about 13,500 votes would be enough to win the contest. The party got 13,209, not far off this target, but it was good enough only for a distant second place.
The Labour total of 19,946 was far more than the SNP expected Labour to get – and far more than Labour expected Labour to get.
"These were voters who came from under the radar. Neither party expected them to vote, or in such numbers," one SNP analyst said.
So the simple political truth is that while Labour and the SNP both got their voters out, the Labour vote was harder, stronger and more solid than that of the Nationalists.
Labour also benefited from the collapse of Tory and Liberal Democrat support – both parties lost their deposits.
Peter Lynch, a senior lecturer in politics at Stirling University, said: "This has left the SNP's plans up in the air. The Glenrothes result was a real wake-up call, but it might do them some good.
"Alex Salmond might have to stop looking so arrogant and he might have to be more responsive to the party, and they might have to focus on the fact that they are now in local government, presiding over a whole series of public spending cuts."
Mr Lynch said many factors would influence the progression towards an independence referendum in 2010, not least the next general election, but the SNP would have been "knocked back" by the Glenrothes result.
But what were the reasons behind Labour's success?
Mr Salmond was adamant yesterday that independence had nothing to do with his party's defeat, nor did he share the widely held view that Gordon Brown's revival as an economic statesman was instrumental.
He said: "Any by-election can further accelerate or slow up the cause of independence, but the cause of independence will be progressed nonetheless.
"We will move on to the next argument, the next political debate, and progress that cause."
Instead, the SNP leader blamed Labour's decision to campaign long and hard on the decision by SNP-led Fife Council to introduce income-based charges for home care.
Mr Salmond said this was the real reason for Labour's advance and he took full blame for the SNP's failure to counter it.
"The failure is of the campaign leadership, which is me, effectively, for not recognising we should have changed to face down a scaremongering campaign," Mr Salmond said.
"That's my fault for not having my finger on the political temperature in the constituency."
Mr Salmond was right, but only in part. Labour did run a focused, local, negative campaign against Fife Council, exploiting the Nationalists' decision to make the council leader their candidate for the by-election. But there were other factors.
It may be an unpalatable political reality for the First Minister, but, by coming to the constituency to campaign, Mr Brown turned Glenrothes into a straight fight between the UK Prime Minister and the Scottish First Minister – and the Prime Minister emerged victorious.
That is not something Mr Salmond would like to admit, nor was he willing yesterday to acknowledge a "Brown bounce".
Labour ran an oppositionist campaign, attacking the SNP at local and Scottish level, forcing the Nationalists for the first time to defend their record in office.
Labour also had a Prime Minister who was local and willing to risk his political reputation by campaigning in the constituency. This helped, as well as his burgeoning image as an economic statesman.
While the people of Glenrothes were swayed by home care charges, by loyalty to their Prime Minister and by a sense of unity in the face of economic turbulence, their decision to back Labour will have a knock-on effect on the cause of independence, whether Mr Salmond likes it or not.
As for the First Minister himself, he insisted yesterday Glenrothes was merely a "setback".
He declared: "While it's certainly true that we had virtually untrammelled political success for 18 months, nothing in politics or life continues in that vein forever.
"The job of confronting a setback is to learn lessons, to overcome it and come back stronger. That's what we intend to do."
Everyone involved in Scottish politics knew the point would come when life suddenly became tough for the Scottish Government, but no-one knew when it would happen. As of 12:45am yesterday, that was no longer a mystery.
What happens now, though, depends on the SNP and how it reacts to this defeat. The party's future, and probably Scotland's too, rests on its response.
He shoulders blame and pledges to 'learn lessons'
ALEX Salmond yesterday took personal responsibility for the SNP's defeat in the by-election, insisting that he was to blame for the unsuccessful direction and strategy of his party's campaign.
Labour won the contest convincingly in the early hours of yesterday morning, retaining the seat with a majority of more than 6,700 over the Nationalists.
SNP managers had been confident of victory right up until the close of polls on Thursday night and the scale of their defeat left many in the party confused as to where they had gone wrong.
But Mr Salmond made it clear yesterday that he believed Labour had won because it had developed a local, "negative" campaign and the SNP had not been quick enough in responding to it.
He added: "Our mistake in my view, and it's a mistake I take responsibility for as leader, was that we should have recognised the threat … and moved to a rapid rebuttal strategy earlier in the campaign."
Despite having visited the area a dozen times in the course of the campaign, Mr Salmond said he wished he had spent more time on the doorsteps to gauge grassroots feeling.
A central feature of Labour candidate Lindsay Roy's campaign had been concerns over an increase in local home-care charges from 4 a month to 11 an hour.
Nationalist candidate Peter Grant is the current leader of Fife Council, which introduced the new charges.
The First Minister had predicted as recently as last weekend that the Nationalists would capture the seat but admitted yesterday he had spoken prematurely.
"I was wrong about the by-election," Mr Salmond said. "We're disappointed with the result. However, we're not disappointed with the campaign we fought. A campaign fought by Labour was a scaremongering and negative campaign but was successful.
"There are lessons to be learned and we will learn them, but Peter Grant and the SNP will be back to beat that sort of campaign in Glenrothes and in other contests across the country."
The issue of home-care charges was not a central concern of voters a month ago, according to Mr Salmond, who said it emerged as a priority in the minds of the public only late on in the campaign.
Gordon Brown took a very different view of the by-election, claiming the win was a vote of confidence for the UK government's response to economic turmoil.
"What I have learned from this by-election is that people are prepared to support governments that will help people through the downturn and offer real help to people," the Prime Minister said.
"They are less willing to support people who have no idea about how to solve the problems we have got."
Jim Murphy, the Scottish Secretary, described the win as a "very strong personal endorsement" of Mr Brown.
Although Labour's majority was down one-third on its 10,660 margin in 2005, Roy increased the party's share of the vote.