Lewis Baston is author of Reggie: The Life of Reginald Maudling and several books about British general elections. He is a consultant on politics, elections and constituencies.

Imagine a situation where support for the parties of government in the United Kingdom suddenly collapses in part of the UK and is replaced by a single, dominant nationalist party that has no interest in forming part of an administration back in London and votes in Westminster in accordance solely with its perception of the national interest of its particular part of the country.

In the light of Lord Ashcroft’s long awaited Scottish constituency polling, this may well be the shape of politics in the United Kingdom for the rest of the life of that particular political construct. That might be a matter of two to five years, or we might have a ‘new normal’ in which the SNP controls a block of 40-50 Scottish seats at Westminster for decades. How would the political system cope?

Let us look at some of the precedents, incomplete and inexact as they are. The most recent case of the politics of a component of the UK going off at a tangent is the collapse of mainland politics in Northern Ireland in the February 1974 election with the victory of Unionists opposed to the Conservative government’s power-sharing agreement.

Because Northern Ireland’s representation is so small and divided between different parties, it is rarely pivotal in Westminster politics. For this to happen, a government has to be on the threshold of majority status anyway, as in 1974-79 or at some points during 1992-97. And, with the exception of Sinn Féin who do not take their Westminster seats, the Northern Ireland MPs pay less attention to their ultimate ends than to pragmatic deal-making. As Ken Clarke remarked in 2010: “in the end you can always do a deal with an Ulsterman.” Enoch Powell, then sitting as a Unionist MP, mordantly observed that the Callaghan government could have won its confidence vote in 1979 for some material concessions to Ulster: “a whiff of gas or a ha’p’orth of tar”.

But now let us jump back a century – to 1874 and to the last time a big block of nationalist MPs was injected into Westminster. On the face of it, the 1874 outcome in Ireland was staggering. In the 1868 election the Liberals had won 66 seats out of 103 there, to 37 Conservatives. But in 1874 the Liberals sank from 66 to 10, with the Conservatives suffering only a small loss (from 37 to 31; two small borough seats were abolished in 1870 for having particularly corrupt elections). The winners were a loosely-organised party standing for Irish Home Rule, which swept 60 Irish seats.

The stated causes of this sudden landslide seem curiously inadequate. The secret ballot made intimidations by landlords more difficult; rural Ireland was disappointed by the limited scope of land reforms; issues had moved away from those (such as disestablishing the Church of Ireland) that united the Liberals and Irish interests. But the landslide itself was not what it seemed. The new party was a very broad one, and had been founded quickly – so there were a lot of MPs elected who were essentially Liberals under a different label, some romantic Irish conservatives and one or two fellow travellers with the violent struggle of the Fenian movement.

At first, the Irish impact was blunted by the inexperience, lack of cohesion and connection with existing parties found among the MPs. But in the parliament elected in 1880 the Irish party became a more organised, formidable force under the leadership of Charles Stuart Parnell. Irish MPs became masters of the filibuster and other parliamentary manoeuvrings, bringing the work of the Commons to a grinding, tedious halt.

In one week in 1881, the House remained continuously in session from Monday afternoon to Wednesday morning. The response of the Speaker and the Westminster parties was to change the way the Commons worked, introducing timetabling measures such as the guillotine and the closure vote; the control of parliament by the Government was inadvertently helped along by the Irish parliamentary struggle.

However, British political alignments over Ireland were not yet fixed, and Parnell could for a while play the Liberals and Conservatives off against each other before the de facto alliance of the Liberals and the Irish nationalists began in 1886. Other parties in similar positions, such as the Catalan nationalists since the 1980s, have maintained freedom of manoeuvre for longer periods and supported governments of each main Spanish party in exchange for constitutional concessions and pork-barrel politics.

If Nicola Sturgeon’s words are to be taken at face value, a large SNP delegation at Westminster would not try to play the parties off against each other but oppose the Conservatives (although Iain Anderson argued otherwise on this site earlier this week), taking us directly to the 1886 stage – though with the SNP’s core ‘ask’ at Westminster less easy to summarise than the Irish demand for Home Rule.

After the 1892 election the Liberals, despite having 40 seats fewer than the Unionists, formed a government with the support of the Irish nationalists, who had splintered into several pieces after Parnell’s scandal but still dominated Irish representation. The Conservative-dominated House of Lords rejected the Home Rule Bill and this pale shadow of a Liberal government faded away in 1895.

The 1892 scenario is probably the ideal outcome of the 2015 election from an SNP point of view, if they are primarily concerned with game-playing to advance the ultimate aim of independence. Propping up a weak government without much legitimacy in England, and making, as Alex Salmond put it in 2008, “Westminster dance to a Scottish jig” would not endear the Union to the English (or indeed the Welsh, whose lack of bargaining power compared to Scotland is a source of resentment). Few dances are an elegant spectacle when one of the partners is not in the mood.

The pivotal role of the (reunited) Irish nationalists was revived in 1910, when they once again had the balance of power and could insist that the Liberals move towards Home Rule again – this time with the House of Lords veto removed. Unionists at the time challenged the legitimacy of Liberal governments such as that of 1910 formed as a result of log-rolling and deal-making; some, such as A.V. Dicey, engaged in extraordinary contortions to reconcile the concept of parliamentary sovereignty with their heated opposition to the validity of votes cast in that same sovereign Union parliament. But it was a fact of political life that the petrified state of Irish electoral politics (a little over 80 seats for the nationalists, a little under 20 for the Unionists) gave a systematic advantage to the Liberals in forming a government, although at a price.

Although the SNP’s MPs quite enjoyed their position during the 1974-79 parliament, when “every night was Burns Night” as the Labour whips kept the show on the road, being a large block is not non-stop fun. The Irish MPs had little to do in parliaments where there was an overall majority, as existed from 1895 to 1910, and the MPs were caught between the pressure from their well-organised party back in Ireland and the seductions of Westminster life.

John Redmond, who led the party from 1900 onwards, was as much a House of Commons man as a nationalist, and was too easily taken for granted by the Liberals. At other times, imprisonment without trial was an occupational hazard for Irish nationalist MPs, up to and including Parnell in 1881-82: British governments responded to unrest in rural Ireland with measures that make today’s frequent anti-terrorism laws look mild.

The SNP is different from the Irish nationalist party, of course. Its ultimate aim is independence, while many of the Home Rulers would have been satisfied with devolution. The SNP runs the Scottish Government, and therefore has power beyond anything the Irish did, although in four fifths of Ireland the nationalists an had electoral and cultural hegemony beyond the SNP’s dreams. There is also general consent in Scotland to being taken as a unit, a political community – there is no prospect even of the most anti-independence areas trying to break away or organising armed resistance. To paraphrase Randolph Churchill, Dumfriesshire will not fight – and Dumfriesshire will be right. But in 1910-22 the British – including the Liberals – were transfixed by the Ulster dimension of the Irish question, and their attitude to the nationalists had more than a touch of cynicism and stringing-along to it.

The SNP is also completely constitutional, while the Irish were the parliamentary arm of a struggle that also involved direct action. The issues of land, development and religion were also entangled with Irish nationalism, and while Scotland has suffered from deindustrialisation there is nothing comparable to the trauma of the Great Famine. The Unionists, and some Liberals, tried to ‘kill Home Rule with kindness’ by reforms to ameliorate the material issues. The Major government tried a version of Balfour and Wyndham’s ‘constructive Unionism’ in Scotland after 1992, but to no avail.

If ever one thinks that devolution has smoothed the path to independence, one has to consider the alternatives. Resisting devolution to the last ditch hardly worked out well, either for Ireland or for British politics, which by 1914 had become poisonous, and extravagant verbal violence risked getting physical (see my earlier piece on this site about the election that never happened – that of 1914).

There is a limited amount, therefore, that 1874-1914 can tell us about what a Parliament with a large SNP delegation would be like. The violence that was not far below the surface of politics back then is thankfully absent even in the rhetoric of modern politics (although an 1892 scenario might wake some dragons). Conversely, it seems unlikely that the situation could settle into normality in the way it did in the late 19th Century, although the SNP may wish to remind its new MPs, however numerous they are, not to get too comfortable down there in Westminster.

Further reading: Ronan Fanning – The Fatal Path: British Government and Irish Revolution 1910-22 (Faber and Faber, 2013)