As the SNP looks set to wipe Labour off the electoral map in Scotland, they appear to be the new "king makers" in a British political world turned upside down.

For now, the Tories would love English voters to believe this. Ed Miliband in the pocket of Alex Salmond and Nicola Sturgeon is a theme designed purely for their core English vote. David Cameron seems willing to exchange an SNP landslide in Scotland for a Tory victory at Westminster.

Coming from the so-called Conservative and Unionist Party, this is an astonishingly cavalier and short-sighted betrayal of the party's history, which they may live to regret. Imagine that, in 1913 and 1914, Unionism was so central to Tory politics that Bonar Law, Scottish-Canadian leader of the Conservative Party, effectively sanctioned rebellion against Parliament if it proceeded with Irish Home Rule.

In the face of this, the Liberals backed away from their third Home Rule Bill, before the First World War intervened. The failure to achieve Irish Home Rule through parliamentary means led to the emergence of a violent, revolutionary nationalism, partition and civil war. This will not happen in Scotland, hopefully. Even so, the Irish question before 1914 does offer a useful, if imperfect historical parallel for our time.

When Charles Parnell's Irish Parliamentary Party (IPP) won 85 out of 101 Irish seats at the Westminster election of 1885, they effectively held the balance of power. Mr Parnell thought that all English politicians could be bought off with votes in the Commons. But, having briefly flirted with a Tory alliance, it became clear that, although the nationalists could deprive both Tory and Liberal parties of an overall majority, they could only put the Liberals in government.

The SNP recently made accommodation with the Tories at the Scottish Parliament. More insidiously, an ironic confluence of electoral interests is emerging around Scottish and English national identity. That may be for the future. After this election, the SNP cannot support a Tory government in London. So like Mr Parnell, the SNP can deny both main Westminster parties a majority, but they can only secure power for one party: Labour.

Despite the similarities between the IPP then and the SNP now, there is a crucial difference that exacerbates the SNP's problem. Whereas Mr Parnell and his successors sought Home Rule ("devo max" in today's parlance) within the Union, for all her appealing overtures to UK-wide progressives (a canny tactic winning votes in Scotland and admiration across the UK), Ms Sturgeon leads a party whose fundamental objective is full national independence for Scotland. She will not entirely rule out a second independence referendum, for good reason.

If we take a step back from the current polls and think about the SNP's strategic options, their position is weaker than it seems. Not only do the SNP have only one key partner at Westminster, they are playing a zero-sum game where, ultimately, the break-up of the UK is the only outcome that matters. Given this, even if they win all 59 Scottish seats on May 7, they will be in an awkward position.

On the one hand, anything that makes the Union appear to work for Scotland will weaken the case for a second independence referendum. That makes SNP support for Labour risky. On the other, if the SNP brought down a Labour government and let the Tories in (as they did Margaret Thatcher in 1979), it would strike a blow to their ostensible left-wing credibility.

Talk of a fundamental realignment in Scottish politics may be premature. If, after May 7, Labour has enough seats to consider forming a minority government, the advantage lies with them. Under the Fixed-term Parliaments Act 2011, Mr Miliband only requires the SNP to abstain on confidence motions. This would not be a "deal" of any kind, and as such we may yet see the Liberal Democrats support Labour.

Make a reasonable fist of governing in a centre-left fashion and hold a constitutional convention that engages Scottish opinion and leads to meaningful further devolution, and Labour still have a chance of saving their Scottish wing from political oblivion, and possibly the Union too.

Dr Michael Collins is a Lecturer in British History in the Department of History, and the Director for Democracy and Institutional Change at the Global Governance Institute, University College London.