May 2015 may go down in history as a more significant date even than September 2014

After all, the referendum result was merely an affirmation of the status quo; this time voters see poised to deliver a resounding Yes to the party of independence, the Scottish National Party. The opinion polls suggest Nicola Sturgeon is about to return a landslide result ending Labour's dominance of Scottish politics once and for all.

A word of caution, however. Those opinion polls - and there have been too many now to count - indicating that the SNP has established around a 20 point lead over Labour are still only that: opinion surveys. We have yet to have the election campaign, and as we know from past experience, things can change very rapidly during Scottish election campaigns.

At this stage before the 2011 Scottish parliamentary elections, Labour had almost as large a lead over the SNP and we all know what happened: it was a landslide for the Nationalists. During those three short weeks, Alex Salmond succeeded in turning the tables - with a little help from the then Labour leader, Iain Gray, and that branch of the Subway sandwich bar into which he took refuge from anti-cuts demonstrators.

Could something similar happen this time? Could Jim Murphy - roles reversed - succeed in pulling Labour back from the brink of electoral oblivion? The hyperactive Scottish Labour leader has certainly made big changes to his party, with a raft of new policies on the NHS, poverty and youth unemployment.

He has shifted markedly to the left on matters like tuition fees and top taxation. Seeking to outflank the SNP as the patriotic party of Scotland, Jim Murphy even took to saying that he was not a "Unionist". Moreover, his core argument that only a Labour government can protect Scotland from a right wing Westminster may still carry sway with many voters.

It's important to remember that at the last Westminster election, in 2010, the Labour party returned 41 seats and won over a million Scottish votes. The SNP returned only six. The Scottish Liberal Democrats returned nearly twice as many as the SNP - 11.

Yet now the opinion polls are suggesting that the SNP could return 40 or even 50 Scottish seats in May. The Nationalists' polling figures have been without precedent. Most commentators, myself included, expected the SNP's massive lead to wilt as we came closer to the Westminster elections. But so far it hasn't.

The SNP now has 100,000 members - four times its membership before the referendum. This is a formidable army of canvassers and party workers. The party is wealthier than ever and has a new leader, Nicola Sturgeon, whose personal satisfaction ratings are higher even than Alex Salmond's used to be. Opinion polls also indicate that more people would vote Yes to independence today than did so in September 2014.

For its part, Labour endures the stigma of having worked hand in hand in Better Together with the Conservatives - who still remain unpopular in Scotland. The Liberal Democrats seem hopelessly adrift in the wilderness of Scottish politics, losing ground to the Green Party.

The referendum seems to have marked a turning point in Scottish political psychology. In the past, Scots were happy to split their ticket: voting for the SNP in Holyrood and Labour in Westminster. In 2010, it still seemed self-evident to many Scots that the best way to keep the Tories out of power was to vote Labour. And of course, many Scottish voters, especially the older ones, had a long history of voting Labour.

But most commentators believe that things have changed since 2010. The idea of sending a block of SNP MPs seems an attractive one to many, who believe that it may be the best way to get Scotland's voice heard clearly. Younger voters do not have the same class allegiance to Labour that their parents had.

Still, Labour's pitch to them on May 7th will be that, in Jim Murphy's words: "the biggest party in Westminster gets to form the next government" and that Scots cannot and should not risk wasting their votes on Nationalist MPs. But as the likelihood of either of the big parties returning an overall majority diminishes, this is becoming a difficult argument to sustain.

The largest party may have tended in the past to become the government in Westminster, but that was when there was a two party system, with Labour and Tories alternating in power. There is nothing in the Westminster rules that says the biggest party gets to be the government.

In our constitution, it is the party that commands a majority of votes in the House of Commons that gets to be the government. And in the age of multi-party coalition politics, that could involve any number of combinations with or without the largest party.

Consider the recent polling projection from the Guardian, which suggests the outcome may be: Con, 274 seats, Labour 271, SNP 53, Liberal Democrat 26, Ukip 4 and Greens 1. Here the Conservatives would be the largest party, and David Cameron would certainly try to form a minority coalition with the Liberal Democrats and perhaps Ukip.

However, it is clear from the arithmetic that Cameron's first Queens Speech would be voted down by a combination of Labour and the SNP. He would not be able to call another general election because of the 2011 Fixed Term Parliament Act, which requires a vote by 2/3rds of all MPs before there can be another dissolution.

It would therefore be left to Ed Miliband to seek to form a government, perhaps with the Liberal Democrats or the SNP - or on his own as a minority relying on SNP support in confidence votes only. We are in a new age of minorities in Westminster and the old Labour versus Tories dialectic is a thing of the past.

This does not mean Scottish voters will necessarily reject Labour; they may still want to 'make sure'. But circumstances have changed. This is going to be the most closely fought general election in my lifetime. And it is going to place unusual demands on voters to understand exactly who and what they are voting for.