MEMBERSHIP of the SNP has officially passed 100,000, giving the party a massive campaigning advantage as it enters a potentially historic General Election.

The vast army of foot soldiers means the SNP heads towards May 7 as the third-largest party in the UK - twice the size of the LibDems, two-thirds the size of the Tories, and half the size of Labour.

The SNP is expected to conduct its millionth doorstep canvass of the campaign this weekend, and deliver its 10 millionth election leaflet since the New Year.

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The latest polls suggest the SNP, which currently has six MPs, could win more than 50 of the 59 Westminster seats in Scotland.

However, senior party figures believe 30 would be a good result, as they expect to be squeezed by Labour and the Tories as voters focus on who they want to be Prime Minister.

Nicola Sturgeon will unveil the latest membership figures when the SNP party holds the biggest conference in its 81-year history in Glasgow next weekend.

Around 3,000 delegates are due to converge on the Scottish Exhibition & Conference Centre, one of the few venues in the country still capable of containing an SNP gathering.

The event will be more than twice as big as any previous Nationalist conference.

Two years ago, at its 2013 spring conference, the SNP was welcoming member number 25,000. Its quadrupling since the No vote in the referendum means one person in 50 in Scotland is now a member of the party, the equivalent of

Labour or the Tories having more than one million members at UK

level - something neither party has enjoyed since the 1950s.

A senior SNP source said: "This is a fantastic achievement - the most significant aspect is that it means we can chap on even more doors, deliver even more leaflets, and talk to even more people in their own communities about the positive case for voting SNP.

"Reaching this milestone sets the tone for what will be an upbeat and above all outward-looking conference in Glasgow."

The latest House of Commons estimates put the size of UK Labour at around 200,000, the Conservatives around 150,000, the LibDems and Greens around 44,000 and Ukip 42,000.

In Scotland, Labour leader Jim Murphy recently claimed his party had around 20,000 members, while the Tories have around 10,000, the Greens 8,600 and the LibDems under 3,000.

Alex Salmond set a pre-election target of 100,000 members in November, when the SNP was 85,000-strong.

Handing over the leadership of the party to Sturgeon, he said: "Let us ensure that by next May's election, the SNP reaches a level which has never been seen before in Scottish politics - 100,000 members representing the national cause."

However, despite an explosion in membership in the weeks immediately after September 18, the pace of recruitment later slowed.

The last time the party revealed its membership was in January, when it was around 93,000.

Next week's conference at the SECC is expected to feature 3,000 delegates, 150 per cent more than the SNP's November gathering, as well as 200 journalists and 42 exhibitors.

SNP election campaign director Angus Robertson said: "Next weekend's conference will be by far the biggest in the history of the SNP - which in turn reflects the record amount of doorstep activity that is taking place the length and breadth of Scotland.

"We have stronger support than going into any previous election. The polls indicate what we can achieve - hard work and commitment will determine what we do achieve, and we are fully focused on winning a strong voice for Scotland."

Professor John Curtice of Strathclyde University's politics department said size wasn't everything in elections, but if properly deployed, a large membership could give the SNP a distinct advantage.

"If you want to knock doors, identify your supporters and get people out to vote then it can make a difference in a tight contest.

"But the more important big number is 18 points, which is the SNP lead over Labour in the polls, and would give them 47 seats to Labour's 10. Given the state of the UK polls, Scotland is now absolutely pivotal.

"Scotland means that the chances of Labour getting an absolute majority look remote Labour is in a tough place and it's Scotland that's doing that."