With Tuesday's announcement in Parliament that ministers would be free to campaign for whichever outcome they personally favour, the Prime Minster fired the starting pistol for an EU referendum race that will dominate this year's politics (and maybe some of next year's too).
It is, as he was keen to stress, up the British people to decide. Like some of the ministers in his cabinet, many voters aren't yet certain which side they'll back in the referendum. Unlike those ministers, many of them don't really care.
The referendum is a big moment for Britain, offering voters a big decision - a choice between two clear and competing options for their country. For such a big decision, however, there is at present a remarkably low degree of public engagement.
It is quite remarkable that for a quarter-century these two competing sides have been fighting a battle among themselves and yet have failed to convince, in many cases even to engage, the wider public. That is particularly true with regard to women and younger voters: one might expect both campaigns to be actively courting such 'up for grabs' voters who are less decided than, for example, older men; and yet the most prominent voices for both 'In' and 'Out' campaigns remain predominantly older and male.
Now both sides in the referendum have no choice but to find a way to appeal to the wider, EU-indifferent public, if they are to win over 50% of the vote to make it over the finishing line. Both sides will need to change their tune: twice as many people currently think 'In' campaigners are out of touch with ordinary people than think they understand their concerns; twice as many think 'Out' has a negative agenda than a positive one.
As British Future points out in a new pamphlet published this week, both sides need to learn how not to talk about Europe - to reach beyond their comfort zone and engage unconvinced voters with their arguments, whether they are for or against Britain's membership of the EU.
The early moves from the campaigns on both sides suggest many of those involved understand that they will need to reach out and to persuade a majority. The tricky bit is actually doing it.
This isn't a general election, with party structures and discipline and political candidates whose careers depend on toeing the line. It will be hard, if not impossible, for the two campaigns to tell their supporters what to say or do.
And the instincts of those supporters may not exactly be in tune with the people they most need to reach - the undecided voters. While most voters might not care very much about the EU or the referendum, the people knocking on doors and organising meetings about the referendum certainly do.
So the winning campaign could well be whichever side does most to disrupt expectations, surprising people with its ability to broaden its appeal. That means both campaigns may need to surprise their supporters too - or at least take them on a journey about how to win the argument with those not already onside.
Otherwise, if and when they get their pitch to the public right, the campaigners might risk a chorus of complaint from their most active supporters, those who would find a narrower pitch more compelling.
Both sides will need to present their answer to the whole of Britain - not just the portion of it that cares most about Europe - if they want to come out on top. It's going to be a big deal, and a big choice - and it looks like it might be close.
'How (not) to talk about Europe' is published by British Future today - read the full pamphlet here.