‘If you hold the balance, then you hold the power.” So says Alex Salmond. But any child balancing on a see-saw summit weighs power against peril. At each end, another child sits at your mercy. Yet if he or she jumps off, it’s you who goes tumbling.

So the SNP must beware if it wins the balance in the next House of Commons. (Still “if”: I know nobody in Scotland who doesn’t privately think the opinion polls exaggerate what’s coming.) Has it thought about the dangers of balancing, as well as the delights? The aim to “lock the Tories out” of power unlocks another question: who would then really be hostage to whom?

This won’t be any formal SNP coalition, but probably something even vaguer than a “confidence and supply” agreement with – presumably – the Labour party. The SNP minority government at Holyrood after 2007 survived from day to day by cunning deals that played the other parties off against one another. They were no match for Salmond’s footwork. To London’s consternation, his government lasted its full term of four years, and Scotland’s voters thought it did pretty well. But temporary alliances with bigger parties can shift – must shift, in fact, if the balance-holder isn’t extracting enough concessions. Otherwise the smaller party will become the prisoner of the bigger one.

All these perils lie open to read in the forgotten story of the Irish Parliamentary party, the “home ruler” MPs who held the balance in the Commons for many periods between 1874 and 1914. Any comparison with Ireland rouses alarm in Scotland, so here come the disclaimers: Scotland was never a colony settled by foreign conquerors; England did not control Scotland by fire and slaughter; Scotland has no Fenian tradition of conspiracy in the cause of independence; and, best of all, Scotland has no political Ulster.

But the SNP has plenty to learn from the home rulers at Westminster. (By home rule, politicians then meant something like “devo max”, well short of independence.) Led by the passionate Charles Parnell, and then by the stolid John Redmond, the Irish Parliamentary party’s obstructionism and filibusters won many reforms for Ireland. But their mistakes eventually destroyed them, and home rule itself – and are still there to be repeated.

Their first mistake was to get too entangled with their allies. In 1886, with 85 seats, they kept Gladstone’s Liberals in power and were rewarded with a home rule bill. But the bill failed, defeated by the Tories and pro-Ulster Liberal rebels. Parnell kept up the alliance even after Gladstone lost the next election. But when Parnell’s secret affair with Kitty O’Shea blew open in 1890, Gladstone disowned him – and the home rulers made the fatal mistake of sacking the charismatic Parnell in order to keep in with the Liberals.

The Irish party split down the middle. Another home rule bill arrived in 1893, but the House of Lords vetoed it. Then followed the party’s second mistake.

From about 1891 to 1905 home rule seemed to go off the boil in Ireland; people agitated instead over land reform and Irish universities. But under the surface, radical discontent was building up. A brilliant, angry upsurge of Irish culture began. Young people grew impatient with the compromises of the home rulers. The Sinn Féin movement emerged, reviving the old dream of full independence, achieved if necessary by force. Redmond and his home rulers missed the significance of this. They turned up the volume of patriotic rhetoric, but waited for the grumblings to blow over. The Irish party was losing touch with its roots.

Here are two warnings for Scotland’s first minister, Nicola Sturgeon, and Salmond. First, how do you play the part of arbiter at Westminster without gradually becoming part of that Westminster system? Both speak openly of their intention to play all-British politics, to use their MPs to support progressive measures that don’t just affect Scotland. That’s noble, but also dangerous for a nationalist party. If you stray wider than Irish or Scottish interests, folks back home will begin to wonder why they sent you to London.

Remember the 'Feeble Fifty' – the great bloc of Scottish Labour MPs who marched south in the 1980s

Remember the “Feeble Fifty” – the great bloc of Scottish Labour MPs who marched south in the 1980s and were simply swallowed up in the clamour of Westminster? If the SNP bloc isn’t to fade away like that, it will have to limit helpful “Britishness” and be obstructive, negative, even disloyal to extort what it wants for Scotland. They will be hated by the other parties in parliament anyway, especially by their unwilling partners, just as the Irish party was hated a century ago. Nothing to lose there.

Second, time is not on the side of a “king-making” minority. A nationalist party in a metropolitan parliament has to deliver, and in the end the delivery has to be self-government. Parnell and Redmond extorted sweeping reforms in both Ireland and Britain. Without the Irish, Asquith would never have broken the Lords’ veto power in 1911. But they didn’t get home rule – not until 1914, when it was too late. By then Irish opinion, which had for years adored them, was disillusioned. People suspected their parliamentarians, apparently resigned to playing by the British rules, had gone native.

It’s a common assumption that Irish independence – the decisive swing of Irish opinion – came about suddenly as a result of the Easter Rising in 1916 and the merciless execution of its leaders by the British. But Roy Foster’s latest book, Vivid Faces, tells a longer tale: that in the previous decades, a revolutionary generation had grown up, bringing about “a preceding sea-change” which would shift Irish opinion “dramatically away from the old constitutionalist home rule idea, and towards a more radical form of republican separatism”.

Who were those young rebels? Could anything like that generation begin to emerge in Scotland? There were few of them, mostly middle class, at first linked by a passionate cultural revivalism – Gaelic language and sports, history pageants, Celtic art and music. Romantic revivalism is pretty rare in Scottish nationalism, which doesn’t do golden ages. But that Irish generation, Foster shows, was also inspired by all the ideologies of liberation storming across Europe and America in those years: utopian socialism, feminism, the cult of nature, the turn to what we would now call “organic” food, crops and dress. A new world of freedom and justice seemed to be dawning. Flinging off parental control, pious conformity and age-crusted “English tyranny” felt like a single glad gesture.

Again, no such moment in contemporary Scotland. Many campaigning lobby groups supported independence in the 2014 referendum, but on pragmatic grounds: a Scottish state would be more likely to enact their reforms.

And yet there was a similar time in 1980s Scotland. When the 1979 devolution referendum failed, politics fell into a coma and the SNP went into deep eclipse. But in the next few years culture exploded into novels, poetry, plays and painting, which were formally unpolitical but tinged with a new and vigorous anger about the condition of Scotland. The SNP has now managed to channel much of this energy into its own “constitutional” politics. But it took the party almost 20 years to do so.

The SNP is an independence party: home rule is a stage on the road to sovereignty. In one way, that makes things simpler. But it also secretes a slow venom, which poisoned the old Irish party.

Time passes; a Labour government is blackmailed into more devolution bills; these fray apart in more exhausting squabbles about powers. Finally, Labour rolls off the see-saw, leaving the SNP helpless. Young radicals start asking: has independence come any nearer? Then could follow the splits in the party, the theatrical defections, the tragic loss of contact between a movement in Edinburgh and a delegation in London. And a “revolutionary generation” in Scotland?

Nothing at present seems less likely. Scots have never been any good at conspiracy. Non-violence is the independence movement’s middle name. Dogged hope against experience is tattooed on its forehead. And yet tremors unthinkable a few years ago are loosening Scotland’s foundations.

In 1914, WB Yeats said that the heroic age was over: “The boy who used to want to die for Ireland now goes into a rage because the dispensary doctor … has been elected by a fraud.” But only two years later, he witnessed his country “changed, changed utterly”. If the SNP does mount the pivot of that Westminster see-saw, it must keep its muscles tense, its eyes and ears wide open.