It has been the most boring question in politics: who do you think is going to win the May election? Only a fool would give an answer. You might as well toss a coin. But suddenly the clouds have parted and there appears clear blue sky ahead. Or is it red? This is courtesy of an interview in the New Statesman by the putative Scottish leader in the commons, Alex Salmond. He says that if the Tories are the biggest minority party, and he holds the balance of power, he would “lock out Cameron” on an early vote of confidence. He would instead support Labour in forming a government.

Statements by party leaders before an election on what might happen afterwards can normally be binned. But the nightmare Tory scenario has them in precisely this position: as the biggest party, stumbling on by begging a majority from a scratch team of Lib Dem, Scottish, Welsh and Irish MPs, as Callaghan did from 1977 to 79. Cameron did offer Scotland more devolved power than Labour ever did. Perhaps this might be remembered with gratitude.

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We can forget all that. Salmond is on the left even of his own Scottish National party. If, as the polls predict, he wins 40 to 50 of the 59 Scottish seats, he will carry an overwhelming mandate of Scottish voters, most of whom previously voted Labour. They can now vote nationalist and expect to secure a favourable Labour regime in London.

This week the Tories denounced Salmond’s declaration as “deeply sinister” and “sabotaging democracy”. But he was following the logic of the Fixed-Term Parliaments Act, passed in 2010 by Cameron. Labour professed to be no happier, seeing Salmond’s move as designed further to subvert its Scottish vote and, in their view, increase the risk of a Tory majority. But Miliband must have been quietly pleased. This means Cameron must secure a majority over Labour and the SNP, treated as a united bloc for the duration.

Unless the polls are seriously awry, that seems unlikely. I never gamble on who gets to Downing Street, but my previous belief that the Tories might rely on Scottish abstention to cling on now looks implausible. British politics is paying the price for centuries of English contempt for the political aspirations of the Irish, Scots and Welsh. Throughout the 19th century Tory (and some Liberal) opposition to even moderate home rule for the “other British empire” ensured a more drastic separatism would eventually triumph.

Ireland was first. The “United Kingdom of Great Britain and Ireland” limped on through Gladstone’s failed home rule bills of 1886 and 1892. The Irish leader Parnell’s version of what today would be called devo max was flatly opposed by the Tories and, crucially, the Lords. Push came to shove when Asquith depended on Irish votes after the 1910 general elections and finally overrode the Lords veto in 1911. The union really came to an end in 1921, partitioned Ulster keeping it nominally alive. Gladstone and Parnell had negotiated a deal on “devo max”, but it was Tory intransigence that made the Irish Free State inevitable.

London learned nothing from the Irish debacle, so history seems doomed to repeat itself in Scotland. Tories and Labour alike were reluctant devolutionists, agreeing only to what amounted to a sop. In Scotland’s case the 1999 settlement was the worst of all worlds. It meant increased control over public services, but overrepresentation at Westminster continued, as did Treasury subsidies to what became “benefits street” economies. Scotland, Wales and Ulster were the Greece of the British union, without the austerity.

The idea that showering power and money on Scotland would somehow assuage separatist sentiment was naive: it encouraged it. Devolution became a boondoggle at England’s expense. During last year’s referendum, when further devolution was promised, there was again no quid pro quo. Scotland kept its MPs and its subvention.

Cameron and Miliband wrongly assumed that those they regarded as kilted hobbits would say thank you and shut up. The rise, fall and rise again of Alex Salmond is Scotland’s response. He knows how to exploit the hamfisted reactions of centrist politicians. He now has power over events beyond anything offered in the referendum campaign. Unlike Nick Clegg in 2010, he can be kingmaker and lawmaker without the bonds of coalition. He can be Nero in his circus, giving the thumbs up or thumbs down to each law, one at a time.

Miliband should be able to keep Salmond happy. An end to austerity, higher taxes and continued borrowing are grist to the mill of most Labour supporters.

Salmond already promises to save Miliband’s Treasury the cost of two of Cameron’s most expensive vanity projects, Trident and HS2. On the latter he is shrewdly demanding it “start in Scotland” and run down the east coast. This is the only sensible route for such a train, running not into the buffers at Euston but through to the continent. It might even get built.

The lesson of separatism across Europe is the same. For restless Ukrainians, Slovenians, Kosovans, Slovakians, Basques and Catalans, regional autonomy is not a passing fad, to be bought off with a few powers and subsidies. It is a visceral response to the arrogance of centralised power. It is the response that many Britons profess towards the overbearing power of Brussels; yet few in Westminster see themselves as the EU of Great Britain.

What, of course, Salmond did not mention this week is his price for sustaining Miliband in office. Will it really be another independence referendum so soon after the last? Or will he move straight to full devo-max – devolved fiscal and monetary authority – as the final step before “independence-lite”?

Miliband should call Salmond’s bluff. He should quickly offer the Scots Parnell’s home rule, warts and all. Salmond could take over his full budget, his welfare state and a currency tied to sterling. He should pay for his own pensioners, regiments and wind turbines. He can gather up his subventions and subsidies, as well as his MPs, and take the lot of them back over Hadrian’s Wall, leaving behind little more than a concept of citizenship and some joint services.

Miliband would have hardly any Scottish Labour MPs to object. The Tories would be out of office and no English voter would bat an eyelid. Miliband would get one parliament, the Tories the next.

• This article was amended on 26 March 2015. An earlier version said incorrectly that “Asquith depended on Irish votes in the 1906 parliament”.