Ed Miliband should treat former Tory prime minister John Major’s demand that he rule out a post-election agreement with Scottish nationalists with the contempt it deserves.

Major should be reminded first of all that his party, with Margaret Thatcher at the helm, could not have forced the resignation of James Callaghan’s Labour government in 1979 without SNP backing.

It was the SNP that first moved a no-confidence vote in the House of Commons, encouraging the Tories to do likewise and to carry the day by a single vote, with all SNP MPs backing the Tory motion.

The guiding principle of the SNP then as now was to “use every strategy it can to break free of the UK,” as Major himself puts it.

It is fully entitled to do so and to seek to persuade Scottish voters to follow this road.

However, what disconcerts Major most is not the SNP doing what it says on the can. It is that the party rejects deals with the Tory Party now.

Unlike 36 years ago, SNP leader Nicola Sturgeon is adamant that, while the Tories are an anathema, it would consider supporting a Labour government in Westminster “on an issue-by-issue basis.”

Many observers see such an arrangement as likely to drag Labour to the left. However, this is not a given.

The greatest problem facing socialists throughout Britain is politicians talking left and acting right.

New Labour made a virtue of sucking up to the City of London, sending its leaders to Mansion House dinners where it told bankers what they wanted to hear and handed public-sector finances over to private firms through public-private partnerships and private finance initiatives.

The SNP suffers from a similar virus, seeking to emulate the Irish Republic by attracting tax-dodging transnational corporations with slashed corporation tax levels.

Plaid Cymru’s latest party political broadcast treads a similar road of promising corporation tax cuts — albeit for smaller companies — and borrowing Gordon Brown’s “Labour means business” line, substituting Plaid for Labour.

Given the SNP stance on corporate taxation, it is surprising that Green MP Caroline Lucas should propose a “progressive alliance” between the two parties.

It’s difficult to see what such an alliance would be expected to do over and above the already existing readiness to vote together on particular issues.

Lucas pointed to common ground with the SNP, and with Plaid also, on non-renewal of Trident, electoral reform and reversing the worst of the austerity agenda.

But much of this common ground is also shared with Labour left-wing backbenchers and Respect MP George Galloway.

Indeed, it forms the basis of the People’s Charter programme of the People’s Assembly, backed by Lucas, the trade union movement and a variety of progressive individuals, community groups and political bodies.

Major’s pre-emptive strike against Labour’s refusal to rule out a deal with SNP — and hopefully with Plaid, Greens, Respect and any other party dedicated to ending Tory-Liberal Democrat rule on behalf of the minted minority — will not be the last effort to encourage disunity.

Britain’s undemocratic first-past-the-post electoral process encourages political convergence and mass alienation.

Progressive candidates elected in May have a duty to work together to press for policies that benefit the people ignored by pro-austerity forces.

But rather than be ensconced in a parliamentary ivory tower, they should build links with people outside Westminster and seek to isolate those dedicated to doing the bankers’ dirty work.