IT IS not yet midday, and the Tolbooth pub has filled with the boisterous green-and-white-clad supporters of Celtic, the football team of Glasgow’s working-class Catholics. But an argument opposite the bar concerns the constitution, not the match. Cathy declares that if Scotland left the United Kingdom it would remain in the EU. Thomas, sipping a neon-orange concoction of Irn Bru and vodka, shakes his head forcefully: “No, we’d have to reapply.” They have one important thing in common, though. Neither has yet decided whether to vote for or against Scottish independence.

On September 18th all Scots over the age of 15 will be offered that choice in a referendum. The pro-union Better Together campaign is reliably ahead in the opinion polls. With a strong hand in any secession negotiations, the unionists have foxed the pro-independence Yes Scotland camp with erudite questions about tricky details. The latest came in a speech by Mark Carney in Edinburgh on January 29th. Though he stressed his non-partisan, “technocratic” role, the governor of the Bank of England warned that for Scotland to share the pound it would need to accept “some ceding of national sovereignty”. On such high-level fronts, the unionists are winning the war.

Yet Scotland’s constitutional future will be decided far from smart conference rooms, in the sort of hard-up neighbourhoods that rarely get much attention from politicians. Working-class Scots are more drawn to independence than are others (see chart). And quite a few of them are still to play for. In most referendums, undecided voters drift into the conservative camp towards the end of the campaign. Scotland’s independence vote may turn out to be an exception. Undecided voters are more left-wing than the average Scot, more hostile to the Conservative-led government in London and more inclined to think that Scotland would succeed alone; in short, they are “almost undoubtedly more favourable to independence,” says John Curtice, a psephologist. They worry unionists keen for a decisive win, and excite nationalists longing for an earthquake. As the Tolbooth crowd settles into the televised match, Yes Scotland’s troops pound the hilly streets outside, enthused by that thought. Like their target voters, they claim to want a kinder, more redistributive Scotland. “A different country with different priorities!” proclaims Mary McCabe, their convener, fists clenched against the cold. Blair Jenkins, chief executive of the Yes campaign, describes the sort of voters he is pursuing as “Labour-type people”, whether or not they currently support the party. Ms McCabe’s gang of campaigners brandish clipboards hinting at how the nationalists plan to win over those Scots. Using a ten-point scale, they rate people’s inclination to vote for independence. After each session, the ratings are fed into a database; campaign bosses use it to monitor changing views. Undecided voters receive electoral bumf—leaflets decrying the London government’s benefit cuts, for example—designed to nudge them up the scale. “People who were three-to-four last year are now around six-to-seven,” boasts Mr Jenkins.

The red machine

Across Glasgow, in Cardonald, a blue-collar district of semi-detached houses and tower blocks, Mr Jenkins’s opponents are trying to nudge people the other way. Standing out from the crowd of sky-blue Better Together jackets (each bearing the slogan “UK OK”) is a dash of red. It belongs to Johann Lamont, the leader of Scottish Labour, which dominates the pro-union campaign. In areas like this people either vote for her party or do not vote at all. So she ought to get a sympathetic hearing. But responses are mixed. One woman with a foam of toothpaste around her mouth admits that, though a Labour voter, she has not decided how to vote in the referendum. She is not alone: according to the large Scottish Social Attitudes survey, 36% of people who identify with Labour are yet to make up their minds.

Better Together’s bosses insist they are not complacent about such voters. Labour still bears the scars of the last time it underestimated the pro-independence camp’s ability to win over working-class folk: the Scottish Parliament elections in 2011. The party ran a lacklustre campaign, assuming few would back the relatively middle-class Scottish National Party (SNP). It was wrong: they swung behind the SNP, carrying it to an unexpected majority.

And the nationalists have a knack for devising pre-election giveaways, such as a package of council-tax benefits a month before the 2012 local elections. One survey in late 2011 found that a small majority of Scots would back independence if it made them £500 (roughly $800) better off. The risk of losing the wallet war is “incredibly dangerous”, says one Better Together figure, pointing to undecided but struggling voters in places like Cardonald.

That fear was reflected in a speech on January 11th by Gordon Brown—part of Labour’s strategy to deploy its big names to win the left vote for Better Together. The former prime minister described the purpose of the union as “the pooling and sharing of resources for social justice”. The SNP responded by attacking not Mr Brown (who is still popular north of the border) but the Tory chancellor of the exchequer, George Osborne.

In the battle for undecided voters, nationalists will try to drag the debate onto the free-market evils of the London government. The Tories, Yes Scotland has concluded, are the best recruiting agent for the pro-independence cause. His eye on undecided voters like Cathy and Thomas, Mr Salmond has repeatedly demanded a televised debate with David Cameron, Britain’s patrician prime minister. For the sake of the union, Mr Cameron should keep declining the invitation.