The first minister was clearly right yesterday to rule out supporting a government that is captive to a separatist party.
Even the Labour Party has ruled out coalition with the SNP, such was the pressure on Ed Miliband over the prospects of Scottish nationalists holding the whip hand at Westminster.
The temptation for Labour to do a deal with the SNP will be great if, as is now highly possible, Alex Salmond leads his fellow separatists to victory north of the border (by being the Scottish party with the most House of Commons seats). Mr Miliband has not ruled out some arrangement with the SNP.
Peter Robinson could not countenance being part of such an understanding. There was major relief among Northern Ireland unionists when Scotland voted by a 10-point margin to reject independence in September, a more decisive victory than was expected by the end of the referendum campaign.
Many commentators said that the scale of the SNP defeat buried the constitutional question for a generation. Even so, there was a degree of scepticism when Mr Salmond swiftly announced that he was stepping down as first minister.
Sure enough, it became clear that his ambition of being the man who dismantled the Union was unabated when he unveiled his Westminster bid. He might emerge as more powerful there than his successor Nicola Sturgeon is at Holyrood.
It would be disastrous if Labour, for reasons of short term gain of office, accommodated the SNP in ways that led to the break-up of the UK.
It would not take much to provoke such a split. Middle England is tiring of the financial demands of Scotland and Northern Ireland (it is troubling that Mr Robinson has made scrapping the overdue ‘bedroom tax’ — better described as the under-occupancy penalty — one of his key demands).
If a minority Labour government promises more largesse to the SNP, the English backlash will cause further damage to the increasingly wobbly foundations of the UK.