LABOUR'S reaction to last week's devastating Ashcroft constituency polls suggesting that they would retain only one seat in Glasgow at the General Election was dispiriting to say the least.

The party issued an election video quoting the party's Scottish deputy leader, Kezia Dugdale, saying: "Fact: the biggest party gets to form the next government #VoteSNPgetTories." The Shadow Foreign Secretary, Douglas Alexander, repeated this word for word on BBC radio.

Yet he knows perfectly well that this is wrong. In our parliamentary system the biggest party does not get to form the next government; it is the party which commands the largest number of seats in the House of Commons, which is very different. Westminster is not an elective dictatorship; it is a parliamentary system of government.

Labour should know this better than most. In 1923, Labour's first ever prime minister, Ramsay Macdonald, formed a government despite winning only 191 seats to the Tories' 258. He governed as a minority with the tacit support of the Liberals. There is no law that the largest party forms the government.

Only in the unlikely event that one of the parties gains an absolute majority of seats (currently 323) in the May 2015 general election, would the biggest party get to form the next government. And this simply is not going to happen. Westminster, like Scotland, is now a parliament of minorities. We are in the age of coalition.

So, even if the SNP had not said they would refuse to support a Tory government, it would still have been wrong to claim that the biggest party automatically gets to form the next government, or that a vote for the SNP is a vote for the Tories. No-one can know how the next government will be composed. All we know is that neither Labour nor the Tories will be governing alone.

I hope and trust Labour will now withdraw this ad (and stop accusing people who try to explain how our system of works as being "in the pocket of the Nats"). It's perfectly reasonable for Labour to say: "Fact. The surest way to get a Labour government is to vote Labour - #acceptnosubstitutes". That is debatable but not misleading. It's also a better copy line.

As it happens, the SNP have said repeatedly, tediously, ad nauseam, that they will never do a deal with the Conservatives. Alex Salmond has said that under no circumstances could the SNP ever help the Tories into government. Personally, I think the Scottish National Party was wrong to have been so categorical about this. In our parliamentary system, it is inappropriate and even undemocratic for parties to rule out coalitions, pacts or electoral deals before seeing the terms on offer.

Sometimes, deals with the devil are necessary in a higher cause. The victorious left-wingers in the Syriza party found they had no option but to join in an anti-austerity coalition with the Independent Greeks, a party considerably to the right of our Conservatives. The SNP relied on tacit support from the Tories in Holyrood in 2007/8.

Labour's VoteSNPGetTories might have worked in the days when the big parties controlled politics. In England the expectation of voters is still very much a binary one: Labour in, Tories out. But the situation here is very different. In Scotland, voters have seen coalition in action in the Scottish Parliament for the last 15 years. And following the referendum they are probably the most educated electorate in Scottish history, after the extraordinary explosion of political engagement that led to 97% voter registration and people queuing up outside polling booths.

Labour are entitled to say that the SNP have form when it comes to not supporting Labour. In March 1979, they withdrew support for the Labour government of James Callaghan and precipitated the election that brought in Margaret Thatcher. Nationalists get very angry when this is raised. They point out that the Liberals under David Steel also withdrew support from Labour, and that anyway the Callaghan government was dying.

This is true. Callaghan made a disastrous miscalculation in not going to the country in October 1978 when he would probably have won. After the Winter of Discontent and the abortive devolution referendum, the Labour government was dead in the water and it was only a matter of time before the Tories took over.

Nevertheless, the 11 SNP MPs in the Westminster group performed the role of midwives (Labour lost by one solitary vote in the Commons division) And many in the party back in Scotland were furious that they had done so. Callaghan's jibe, that they were "turkeys voting for an early Christmas", was dead right. The SNP lost nine of its MPs in May 1979, split and plunged into electoral obscurity for the next 20 years.

Will they be any the wiser this time? Almost certainly. The SNP today is a different party from the rag bag of ideologically promiscuous outsiders who were propelled to prominence in the 1970s by the economic crisis and the lure of Scotland's newly discovered oil. The party today is a social democratic, tried-and-tested party of government with a highly professional organisation and a mass membership.

In the 1970s, there were constant disputes and struggles between the prima-donna MPs in Westminster and the party organisation in Scotland. Now it has a clear leadership structure and a forceful leader in Nicola Sturgeon. It's possible that Alex Salmond, his ego inflated by media attention in London, could become the back-seat driver from hell - but I don't actually see it happening.

However, the demands on the party this time in a hung parliament could be great, and its democratic responsibility onerous. The Lord Ashcroft constituency polls suggest that the SNP has taken Scotland by storm; the map is now Nationalist yellow. British politics could be turned upside down as a contingent of 40+ nationalist MPs become a decisive factor in the future governance of the UK. The SNP will have real power.

It could, like Charles Stewart Parnell, the Irish home ruler in the 19th century, play havoc with Westminster by cutting deals left, right and centre; playing Tory off against Labour to its own narrow advantage. It could use its electoral leverage to secure further devolution. Remarkably, the SNP has chosen not to do so and make clear that the only party it will play politics with is Labour.

Nicola Sturgeon has thus handed an extraordinary advantage to Ed Miliband. He knows the SNP will go into post-election negotiations with precisely zero negotiating clout. Indeed, Labour doesn't need to form any coalition with the SNP at all because they have ruled out voting with the Conservatives. The SNP will not be king-makers in a hung parliament, they will be vassals: wholly-owned property of the Labour leadership. Nicola Sturgeon's red lines are purely hypothetical because Ed can reject them all and say: OK, and are you going to do about it?

Labour in Scotland should be careful what they say in case the SNP change their minds on this. As things stand, Ed Miliband is in a poker game with the SNP which he can't lose. The only honest hashtag has to be: "#VoteSNPgetLabour."