Party chiefs insisted they were in an "upbeat mood" as they waited for the polls to open today in the crucial by-election.
One senior source said: "We will be out knocking on doors until people stop answering."
The source added that Lindsay Roy, the party's candidate, would give the contest one final push, because Labour had "narrowed the gap so much that we are within striking distance".
However, that appeared to be a tacit admission that the Nationalists were ahead going into polling day – and that was reflected in Alex Salmond's bullish decision to put his own money on an SNP win.
The First Minister has seized on Barack Obama's campaign slogan, "Yes we can", and he insisted yesterday: "Yes, we can win and, yes, we will win the Glenrothes by-election."
Asked whether he had "put his money where his mouth was", Mr Salmond replied: "You know I never comment on my gambling propositions, but if you publish that I put money on the SNP to win, I won't deny it."
SNP managers believe they have "won the ground war" by getting more activists than Labour out in the constituency, although some concede privately that Labour may have won the "airwaves war" and have had a better campaign in terms of publicity and message.
However, the Nationalists believe that the personal contact of so many SNP activists with so many of the electorate will tell when the votes are counted tonight, with the result expected early tomorrow morning.
Victory would give the SNP its second Westminster by-election victory of the year, following its success in Glasgow East in the summer, and its second successive defeat of Labour. A victory in Glenrothes would be a massive boost for the Nationalists, who have set themselves a target of having 20 MPs after the next general election.
It would also fuel the momentum Mr Salmond and his administration have maintained since they won the Holyrood elections in May last year.
Defeat for Labour would be devastating for the party, even more so than in Glasgow East.
The Glasgow by-election came when Prime Minister Gordon Brown's personal popularity was sinking to damagingly low levels all over the country and Labour was torn apart by factional fighting and constant rumours of leadership challenges.
But the Glenrothes contest has been fought under much more favourable conditions for Labour. The Prime Minister has been to the constituency twice, boosted by his performance as an international statesman during the banking crisis.
His wife, Sarah Brown, has been in the constituency many times, and the voters of Fife are at least expected to be much more sympathetic to Mr Brown, a fellow Fifer, than the people of Glasgow East were. Yet if, after all these advantages, Labour loses today's vote, it will send out warning signals to every part of the Labour Party that it is in serious trouble ahead of the next general election.
Maurice Golden, the Conservative candidate in Glenrothes, said the best way to send Mr Brown "a message" was to vote Tory, although he admitted it would be an "uphill battle" for him to win the seat, which Labour is defending with a 10,664 majority over the SNP.
Harry Wills, the Liberal Democrat candidate, was joined by Tavish Scott, the Scottish party leader, and Willie Rennie, the Dunfermline and West Fife MP, as he campaigned yesterday, concentrating on the economy and the party's plans for a 2p cut in income tax.
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