One of the most effective criticisms supporters of the Union make of the SNP is that it offers only a vague or incomplete picture of what an independent Scotland would actually look like. Take as an example the party's defence policy. It is still far from clear exactly how many troops a Scottish Defence Force would have, how it would be structured and what kind of budget it would be run on. By refusing to provide absolute clarity on this issue, the nationalists are helping to fuel a widespread sense of unease at the prospect of radical constitutional change and, consequently, diminishing the likelihood of the independence referendum returning a majority Yes vote in 2014.

Unionist politicians know how serious a problem this is for the SNP. As Alistair Darling has done recently, they will try to use it as a way of promoting the idea that secession amounts to a dangerous and reckless leap into the unknown. It is odd, then, that on one of the defining issues of modern Scottish politics, Scotland's main unionist party - Labour - seems incapable of providing any clarity of its own.

The question of whether or not Scotland should continue to allow Trident, Britain's Clyde based nuclear-armed submarine fleet, to be stationed in its waters is of enormous significance. In addition to the massive cost associated with its replacement and maintenance (estimated at £100bn over the course of the next three decades), it represents a serious risk to Scotland's population and environment, as a 2009 report into the myriad safety failings at the Faslane installation revealed. Further, in 2010 YouGov published a poll which showed that nearly 70 per cent of Scots were opposed to the renewal of Trident. This gives the SNP, which has always favoured unilateral disarmament, a real political advantage as the referendum approaches.

Yet in their speeches at the Scottish Labour conference in Dundee earlier this month, neither shadow defence secretary Jim Murphy nor shadow foreign secretary Douglas Alexander made any reference to Trident whatsoever. Instead, both chose to defend the proposition that Britain plays a positive role in global politics, with Murphy even boasting about the UK's bloated military budget. What's more, the new Scottish Labour leader Johann Lamont also omitted to mention it, as she has done consistently since being elected in December. In fact, during last year's leadership contest she was the only one of six candidates (including those standing for the post of deputy leader) who declined to respond to a letter from Scottish CND on the subject.

Two things account for Lamont's vow of silence. The first, as her voting record in the Scottish Parliament suggests, is that the Trident question seems to throw her into a state of abject confusion. In 2003 she supported a motion put before the chamber by Tommy Sheridan which described nuclear weapons as "a very real threat to humanity" that should "be opposed on moral, political and economic grounds". Yet in the same parliamentary session she also voted against another motion asserting that "there is no justification for the renewal or replacement of the Trident nuclear weapons system". In 2007 she again voted against a motion - this time put forward by Nicola Sturgeon - in opposition to the replacement. But then, quite bizarrely, she abstained from a vote on Patrick Harvie's motion congratulating his fellow MSPs for having condemned Trident.

The second is the stance of Labour's UK leadership, whose support for Trident (Ed Miliband and Ed Balls both voted for renewal in the House of Commons in 2007) leaves no room for dissent at the top of the Scottish party. That is, even if Lamont personally favours abolition (as some suspect she does), she is unable to say so because it would cause a hugely damaging rift with her Westminster superiors - and given Scottish Labour's traditional relationship with its London HQ, that is not something she is likely to provoke.

But Lamont cannot stay mute indefinitely. At some point, presumably before the referendum debate really heats up, she is going to have to voice her opinion: for or against. If she doesn't, not only will she sacrifice a sizeable chunk of political credibility, but the Labour-unionist charge that all the risk and uncertainty lies with the SNP and independence will begin to look desperately hypocritical.