The Nationalists put 1,200 activists into the constituency over the weekend to campaign on behalf of Peter Grant, the SNP candidate.
Party managers had hoped for 1,000 volunteers, but the extra helpers meant they could leaflet the entire constituency in a single four-hour push over the weekend.
Alex Salmond then increased the pressure on Labour by insisting the SNP would win Thursday's contest.
He said: "I don't anticipate we shall win by a landslide. But you ask me the question, 'Do I think we will win?' The answer is ,'Yes, I do'."
The First Minister argued that the Nationalists had better organisation on the ground.
Labour hit back, however, with John Park, the party's campaign coordinator, warning Mr Salmond that voters would not be taken in by his pre-election "bluster".
Mr Park said: "People don't like being told by politicians how they're going to vote in advance. It's typical of the First Minister that he takes people's votes for granted and he obviously thinks this is in the bag. It isn't, and the people of Fife are not going to be fooled by bluster."
Labour is also expected to bring in MPs, MSPs and councillors from around the country for the last few days, while Sarah Brown, the Prime Minister's wife, visited yesterday.
A Labour spokesman said: "She's just quietly been talking to people on the doors and getting on with it."
Labour has also started to target the record of the SNP-led Fife Council, focusing on charges for pensioners and fears over the introduction of a local income tax.
Lindsay Roy, Labour's candidate, said: "Local people are worried about what local income tax will mean for them. With so many people opposed to their plans, the SNP should have the humility to scrap them.
"At this difficult economic time, the last thing anyone needs is a new tax."
Meanwhile, the Nationalists are going into the final few days of the campaign concentrating on post office closures.
There have been unconfirmed reports that the UK government is about to award the contract for the Post Office card account to PayPoint, a decision which could deprive post offices of 20 per cent of their business and drive many to closure.
The SNP claims five post offices have already closed in the Glenrothes constituency and has accused the UK Labour government of delaying the decision on the Post Office card account until after the Glenrothes poll.
Mr Grant said yesterday: "People in Glenrothes can show their determination to keep the Post Office open with a vote for the SNP in this by-election."
The contest in Glenrothes was triggered by the death of Labour MP John MacDougall, who had a majority of 10,664 at the last election.
Bookmakers William Hill has Labour and the SNP as joint favourites to win, on 5/6.
Cleverest children 'grow up to be Green'
THE brightest children go on to vote Liberal Democrat or Green, according to a survey.
The study by Edinburgh University researchers has found that childhood intelligence is linked to voting preferences and political involvement in adulthood.
The study – which looked at voting patterns in the 2001 general election – found that those with higher IQ ratings were more likely to vote Lib-Dem or Green in an election.
The survey – including cognitive tests at ages five and ten – was followed up with a study of voting habits at 34. Those who voted Green had an average IQ of 108.3, with Lib-Dem voters just behind at 108.2.
Conservative and Labour voters were further behind – with scores of 103.7 and 103.0 respectively, while voters for Welsh nationalist Plaid Cymru scored an average of 102.5. Scottish National Party voters had an average IQ of 102.2. The research also showed that British National Party voters had the lowest average intelligence – scoring just 98.4. Non-voters were found to have an average IQ of 99.7.