Scottish First Minister Nicola Sturgeon's demand for a referendum do-over got less attention than a Spanish court's decision the same day to ban a prominent separatist politician from seeking elected office. But the latter underscored why Ms. Sturgeon's main reason for wanting a second vote on independence – so that Scotland can remain part of the European Union – does not hold water.

For EU countries grappling to tame their own separatist movements, the prospect of an independent Scotland gaining access to the union is about as palatable as a plate of leftover haggis. Their own unity could not withstand the precedent set by letting Scotland join the EU.

Ms. Sturgeon's contention that, by holding a successful second referendum before Britain officially leaves the EU, Scotland could simply remain part of the bloc doesn't fly, either. EU officials have made it clear that an independent Scotland would have to negotiate its entry into the union, a years-long process that would be subject to a veto by member countries.

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Spain, for one, would face no choice but to exercise its veto. A showdown between the central government in Madrid and the Catalonian regional administration in Barcelona is looming once again, as the separatist coalition governing Catalonia vows to hold an independence referendum by September. Madrid insists that an independent Catalonia would be forever excluded from the EU.

Globe editorial: Nicola Sturgeon's call for another Scottish referendum is crass opportunism

No sooner had Ms. Sturgeon announced a second referendum by early 2019, than Spanish Foreign Affairs Minister Alfonso Dastis warned that an independent Scotland would have to "get in line" if it wants to join the EU. "Spain will encourage no secessionist movement in Europe," he said.

As Mr. Dastis was making his comments on a visit to Peru, a Spanish court was handing down its sentence to Artur Mas, the former Catalonian regional head, after the latter was found guilty of "disobeying" the country's constitutional court by holding an illegal independence referendum in 2014. The confusion surrounding that plebiscite meant that only about a third of Catalonia's 6.3 million eligible voters turned out, though about 80 per cent of them voted to leave Spain.

Mr. Mas was banned this week from holding elected office for two years. But he had already ceded the movement's leadership to the current president of the Generalitat of Catalonia, Carles Puigdemont, who heads a coalition government of separatist parties. Mr. Puigdemont vows to adopt a series of laws governing an independent Catalonia this spring in advance of a Sept. 17 referendum.

The central government, led by Prime Minister Mariano Rajoy, still insists such a vote would be illegal. But Ms. Sturgeon's move to put Scottish independence back on the front burner will encourage her Catalonian counterparts to press their democratic claims, especially if British Prime Minister Theresa May agrees to Ms. Sturgeon's demand to hold another referendum.

All of this creates yet more indigestion for EU leaders struggling to keep their increasingly besieged club intact amid nationalist and populist uprisings in the Netherlands, France and elsewhere. While Ms. Sturgeon's move might be seen as a vote of confidence in European integration, it actually complicates the job of those seeking to keep the EU from imploding.

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And for what? While Britain's decision to leave the EU has somewhat bolstered support for Scottish separatism – more than 60 per cent of Scottish residents voted to remain in the EU in last June's referendum – low oil prices have undermined the economic case for independence. Ms. Sturgeon has neither a good economic hand to play nor an answer to bread-and-butter questions about an independent Scotland's borders (open or closed) and currency (pound or euro). Such unknowables would cool enthusiasm for taking the separatist leap as any referendum date approaches.

What Ms. Sturgeon does have is a window of opportunity to pressure Ms. May to move off her "hard Brexit" position and accommodate Scotland's demands by threatening another referendum. But the Scottish leader also risks seeing her gambit badly backfire.

As Telegraph columnist Daniel Capurro noted the other day, if the Scottish separatists lose a second time "that will be it, for good. If the losing margin is bigger, Sturgeon will look like a fool. But if it is a tighter result, Quebec shows that this exhausts people. It makes them reluctant to return to the question and put up with more debate and uncertainty."

Scottish separatists are likely to discover, as their Quebec counterparts have, that you can never win a neverendum.