A public vote offers the best prospect of responding to democratic alienation from the union, and establishing a secure platform for the UK's engagement in its future.
His party wants a eurosceptic but the PM may decide that he needs a business figure with a record of constructive engagement with Brussels.
UKIP trails Labour by six points but, as in 2009, the party is hoping for a late surge in the polls.
Promising an in/out vote would shift the debate back onto Tory territory and could wreck a future Miliband premiership.
While the more established parties, such as the Front National and the Sweden Democrats, look set to enjoy the next year, others are likely to remain firmly on the fringe.
Promising an in/out vote would shift the debate back onto Tory territory and allow Cameron to claim that Miliband is dancing to his tune.
The threat by Nissan to withdraw from Britain if the UK leaves the EU shows how the Tories' euroscepticism pulls against their emphasis on stability.
Such a clear U-turn would cement a corrosive narrative that could prove far more damaging to his prospects of becoming Prime Minister – that of weakness.
The EU doesn't even make it into the top ten of voters' concerns. Miliband's speech should focus on housing, wages and jobs.
Many opposition MPs are persuaded by the case for making a virtue of necessity: resolve the issue and expose Tory divisions.