As Labour gains ground with its cost-of-living offensive, the Tories have sought to present themselves as the "grown-up" party that can be trusted to maintain economic stability and avoid short-term "gimmicks" that threaten this aim. But this strategy risks being undermined by their decision to raise the spectre of EU withdrawal. On the day that MPs debate Conservative MP James Wharton's bill guaranteeing a referendum in 2017 (and Adam Afriyie's amendment for a vote in October), the head of Nissan has warned  that his company could withdraw from Britain if the UK leaves the EU. Carlos Ghosn said: "If anything has to change we [would] need to reconsider our strategy and our investments for the future." Were Nissan to leave, 6,500 jobs at the company's Sunderland site would be lost.
For the Tories, who have made much of the renaissance of car manufacturing  in Britain (with output forecast to reach a record high by 2015), it's an awkward message. Of the 30 brands manufacturing 70 models in the UK, Nissan is the largest and recently announced that its Sunderland plant would move to 24-hour production in 2014 to meet demand. With the uncertainty now set to endure until at least 2015, Ghosn's warning is likely to be the first of many that tarnish the Tories' economic brand.
And for what gain? Those who confidently predicted back in January that Cameron's EU referendum pledge would shoot Farage's fox, or even set the Conservatives on the road to victory, have been proved entirely wrong. The motivations of those who support UKIP are too complex and long-term for them to be bought off by the promise of a vote in 2017.
While the public share the Tories' euroscepticism, they do not share their obsession with the subject. As polling  by Ipsos MORI has consistently shown, voters do not regard it as one of the ten most important issues. It's true that they overwhelmingly support an EU referendum but as pollsters regularly attest, this merely reflects their general predilection for such votes.
Cameron knows and understands all of the above. One of the principal aims of his speech was to settle the debate, calm his restive backbenchers and move on. But with the focus now likely to shift to exactly which powers he will seek to repatriate, there is little prospect of any relief.