When Theresa May was in Brussels last Friday finalising an interim Brexit deal, Jeremy Corbyn was in Switzerland speaking at the UN’s Geneva headquarters. As British politics was consumed by battle over the terms of EU departure, it is fitting the Labour leader happened to be in the country famous for avoiding European conflicts.

Corbyn’s Brexit stance looks a lot like neutrality. It is not an issue that ignites his campaigning spirit. His power to mobilise legions of loyal supporters was not deployed for the remain cause in the referendum. It has since been marshalled towards other causes. In parliament, the tone of Labour’s response to May’s European manoeuvres sounds more like commentary than combat.

John McDonnell revealed the essence of his party’s convoluted Brexit calculations when he said on Monday Labour prefers to talk about “a single market” as distinct from “the single market”. There is only one continental single market under discussion in the current negotiations: the UK can be in it, as most Labour MPs would like, or out of it, which is the government’s goal. The shadow chancellor’s indefinite article clarifies only his reluctance to make definite choices.

Corbyn’s roots are in the vintage left Eurosceptic tradition, but his fortune has been made by a new, instinctively Europhile generation

Such calculated ambiguity has served Labour well so far. The party fought the June election with a manifesto that was explicitly but not ostentatiously pro-Brexit, and a campaign that steered debate elsewhere. Labour reassured leavers in its traditional heartland seats that it would not undo the referendum result, while scooping up votes from remainers whose urgent motive was thwarting May. The electoral alliance that deprived the Tories of a majority is a patchwork of anti-Conservative feeling, masking contradictory attitudes towards the EU and the Labour leader.

Even within the pro-Corbyn camp there are latent tensions between culture warriors who despise Brexit as Trumpesque xenophobic nationalism, and ideological stalwarts who have always seen the European project as irredeemably capitalist. That is largely a generational divide. Corbyn’s roots are in the vintage-left Eurosceptic tradition, but his fortune has been made by a new, instinctively Europhile generation – the crowd that chanted his name at Glastonbury. The Labour leader doesn’t want to betray their affection, but nor does he comfortably deviate from familiar doctrines.

For McDonnell the calculation is different. He sees EU rules that promote market competition and prohibit some forms of industrial subsidy as impediments to Labour’s renationalising ambitions for the economy. But he also wants to be an unthreatening candidate for the chancellor’s job. He recognises that businesses and investors like the single market, and the Treasury needs the revenues that accrue from membership.

McDonnell is pragmatic enough to dial down his distaste for European institutions if that helps defuse the City’s terror of a Corbynite government. Besides, pursuing a radical economic programme while aligned with EU rules might be inconvenient, but not as inconvenient as trying to pursue a radical programme in the crater of an economy that has been struck by a meteor-hard Brexit.

Opaque signals on Brexit are symptomatic of a deeper problem for the Corbyn project. It draws its insurgent energy – its political life force – from promises of unalloyed radicalism. But that was easier to sustain when government felt remote.

The relative success of Labour’s election campaign took even its authors by surprise. Labour’s 40% vote share contains a portion who backed the party in spite of its leader, not because of him. Escaping opposition means reassuring those sceptics, inspiring loyalists and making new recruits. Those agendas are not easily combined.

It is tricky, for example, to persuade investors that their money would be in safe hands under Labour, while also pushing Facebook clips in which the leader tells bankers: “You’re right – we are a threat.”

It is possible that the incumbent administration is so lousy it is destined to collapse, making Corbyn prime minister by default. But there are no guarantees. May could hardly have shed more authority in the past six months had she been trying to sabotage her electoral prospects; yet she approaches the end of the year with some semblance of credibility restored. The deal that allows her to proceed to the next phase of Brexit is gooey fudge, but her MPs are swallowing it.

She is now able to trumpet progress to the sizeable chunk of the electorate that just wants to know Brexit is happening without being bothered by fiddly details. That attitude happens also to sum up the Labour leader’s approach. Even allies concede that Corbyn is not interested in the fine grain of trade analytics.

Responsibility for the technical stuff has been delegated to Keir Starmer. The shadow Brexit secretary has navigated Labour towards advocating maximum alignment with the single market. But his strength is lawyerly forensics, not motivational politics. That is the leader’s department.

Facebook Twitter Pinterest The Labour leader on the Pyramid Stage at Glastonbury in June. Photograph: Oli Scarff/AFP/Getty Images

As a result, Labour’s stance looks both vague and pernickety – focused on the terms and conditions without describing the actual product. This has dulled attacks on a prime minister who doesn’t appear to know what she is buying but ticks the T&Cs box regardless.

Labour cannot lurk in the shadow of Tory disunity forever. Even if the Conservatives maintain their incompetent streak, the negotiating timetable for the second phase of Brexit will impose stark choices on Downing Street. Corbyn will need a response. But the opposition has come to rely on ambiguity as a way to avoid confronting other unresolved issues. It is a curtain from behind which the party emits different signals to different audiences: one message for older leavers, another for young remainers; a radical pitch for some, a pragmatic tone for others.

This system works as long as the government is also hiding from tough decisions. But May’s Brexit path will soon get clearer. Then Labour will have a duty to the country to stop its European prevarications. The options will be fewer and simpler: follow or lead.

• Rafael Behr is a Guardian columnist