AFTER Glasgow, Dumfries, Stirling and Inverness, today is Dundee’s turn to play host to thousands of marchers for independence.

Many will be SNP members. Local SNP MPs Stewart Hosie and Chris Law are among those due to attend. But it is not an SNP event.

Instead, it has been organised by the All Under One Banner group, partly out of frustration that Nicola Sturgeon is not being forceful enough in pursuing another vote.

The tensions are such some Yes supporters even plan to demonstrate at the SNP conference in October to exhort the First Minister to use the ‘triple-lock’ mandate she says she has to call a referendum by 2021.

At the last SNP conference in June, Ms Sturgeon tried to dampen down calls for a fresh vote, saying the focus should be on converting No voters, not the date. “Let’s stop obsessing all of the time about when we might get the chance to vote on independence again,” she said.

But the obsession never went away. Nor is it really an obsession. It is a completely reasonable thing for Yes supporters to contemplate given that mandate has a limited shelf-life.

But as the former SNP cabinet minister Alex Neil argued this week, the game’s a bogie. Theresa May will never give the Scottish Parliament the power to hold a referendum.

She blanked Holyrood’s request for a so-called Section 30 order before the last general election.

She would certainly do so again, not least because she is now reliant on the ultra-Unionist DUP to stay in power. Even if she were toppled, no new Tory or Labour PM would be any keener to add a fight over Scottish independence to an in-tray already bursting with Brexit.

Impatient Yessers may argue for a unilateral, Catalan-style referendum, and to hell with a Section 30 order.

But besides looking desperate, such a vote is open to legal challenge. A Catalan-style referendum wouldn’t end in independence, it would end in the courts.

Given the improbability of a Section 30 order, it falls to Ms Sturgeon to manage expectations and prepare folk for an anti-climax.

So far she’s kicked the can down the road. After being blindsided by the last election, she “reset” her March 2017 plan for a Brexit-related referendum and promised a new “precise timescale” this autumn.

Now, with Brexit still a hazy mess, she has hinted she might not have enough information to say anything concrete this autumn after all.

But while kicking the can is a time-honoured political device, Ms Sturgeon is fast running out of road.

There are snap elections, but no snap referendums. Each referendum is governed by its own particular Act of parliament, and that takes time.

There also has to be six-month gap between the legislation and the vote to allow for lead campaign groups to be set up and for a campaign period.

The last independence referendum was governed by a 2013 Holyrood Act which started life a full 18 months before the vote.

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Ms Sturgeon published a draft bill for another referendum in October 2016 for a three-month consultation. It is essentially a list of tweaks to the 2013 Act, which was to be used as a template. But it wasn’t introduced at Holyrood, so would still face a parliamentary passage of around nine months - opposition MSPs would be sure to take their time mulling all of the 100 or so variations to the 2013 Act.

The Electoral Commission says a referendum cannot be the same day as the Holyrood election, so we need to work backwards from there to estimate the theoretical deadline for calling one in this parliament.

It would not be spring 2021 in case it became entangled with the Holyrood vote. In 2017, a Tory surge in the local election rolled into a Tory surge at the general election. Ms Sturgeon would not want to see a No vote in a referendum snowball into a rout for the SNP at Holyrood.

So the latest practical date for a referendum is likely to be autumn 2020. But, critically, this would have to be preceded by 15 to 18 months of legislation and campaigning. That means Ms Sturgeon would have to start the ball rolling by the middle of next year to hold one, in theory.

So if, by summer 2019, she hasn’t called for a second referendum (albeit probably in vain), it will be obvious to all that it isn’t happening.

If she pretended otherwise and insisted it was still a live possibility she would be accused of treating people like fools who couldn’t read a calendar or do basic arithmetic.

Similarly, if she demanded a referendum at the end of the electoral cycle, it would be seen as a naked gimmick to gee up support for the SNP going into 2021.

She could, in theory, activate her draft referendum bill sooner without actually, calling for vote, introducing it to parliament as a ‘contingency measure’ if needed.

She would have to leave the date blank, however. Theresa May tried that with the EU Withdrawal Bill, which initially referred to “exit day”. But after much criticism about this being weak, dodgy and ridiculous, the government included the date. If Ms Sturgeon tried the same trick, she would risk the same accusations of weakness, flakiness and abuse of process.

She would also be reminded Alex Salmond named the day of the 2014 referendum when he introduced the associated bill at Holyrood.

The First Minister’s headache is that if she comes out and declares a second referendum dead in this parliament, she risks antagonising an awful lot of Yes supporters who were counting on her to deliver it. Some might even have believed that ‘triple-lock’ mandate stuff.

There would be recriminations, accusations she let the side down, gave up too soon, squandered her chance, blinked first against one of the weakest PMs in British history.

Internal party discipline would fray. It would also be hard to go into 2021 asking for a fresh mandate after admitting she couldn’t cash in the last one. But pretence risks antagonising people too. SNP members would not appreciate being left looking daft on a hill they had followed her half- way up.

Coming clean is the better option. It’s not a good look for a leader, especially one proposing a bold new destiny, to let drift and inertia make the decision for her. A shrug and ‘Oh, would you look at the time?’ isn’t quite as inspiring as ‘Freedom!’

She can’t keep everyone happy. But that’s life. The crunch is coming, and it’s coming fast.