It must have seemed like a good idea at the time.
When David Cameron was finally persuaded to back an in/out referendum, it was designed to shoot the UKIP fox ahead of the 2014 European elections. It didn't.
It must also have seemed like a fairly safe promise to make, when just about everyone thought the best the Tories could hope for was another coalition government after 2015.
The Lib Dems could be relied on to block the vote.
But as soon as David Cameron won a majority in May, the referendum became a reality.
Now, suddenly, a Prime Minister who looked completely in control of events is at their mercy.
All day, Tory MPs are declaring their positions - and there are far more "Leavers" than Mr Cameron was expecting.
He faces an awkward couple of hours of questions in the Commons this afternoon.
The Leavers are cock-a-hoop about Boris Johnson's announcement. It has given their campaign much-needed early momentum.
But David Cameron is at his best when his back is against the wall. Remember last May. Remember his leadership campaign in 2005.
His leadership is surely on the line now. Few people at Westminster think he could stay if the country votes to go.
His future depends on June 23rd. More importantly, so does the future of our country.