The Conservative Party’s victory in the British general election means that soon it will fulfill its promise to hold a referendum on Britain’s continued membership in the European Union. The plan will be announced on Wednesday at the opening of the new parliament–during the Queen’s speech, traditionally the centerpiece of the event.

Why has Cameron promised a referendum on EU membership?

David Cameron promised that the referendum would come after renegotiations with Britain’s EU partners. He began that process at last week’s EU summit in Riga, Latvia’s capital, and has embarked on a week of shuttle diplomacy, with visits to several EU capitals.

When Cameron became party leader in 2005, he urged his party to stop “banging on about Europe.” But when the Conservatives returned to power in 2010, many of its parliamentary members were again preoccupied with the EU.

In the first two years of the Cameron government, 93 Conservative MPs (30 percent of the parliamentary party) rebelled on at least one of 29 votes on European integration issues. A fifth of those rebelled 10 times or more.

Many of those Conservative rebels would like to see Britain leave the EU. That agenda has been fueled, in part, by the rise of the United Kingdom Independence Party (UKIP), which advocates withdrawal.

UKIP won more than a quarter of the vote (26.6 percent) in the elections to the European Parliament last year. Even though UKIP won just a solitary seat in Britain’s parliament–an artifact of the electoral system–the party won nearly 4 million votes or 12.6 percent of the total cast in this past month’s general election.

UKIP won that large portion of the vote in no small part because of its charismatic leader Nigel Farage. Although detested and frequently mocked by what Farage calls the  “liberal metropolitan elite,” UKIP and its leader has helped set the Euroskeptic tone of much of the debate through “issue capture,” as I explain a paper authored with Nathaniel Copsey.

UKIP’s strategy is directed at both disaffected Conservatives in the south of England and disillusioned Labor Party voters in the working class areas of northern England. The party shrewdly did not focus on the EU but on the tangible consequences of Britain’s membership.

In particular, UKIP linked one of the EU’s core principles, the free movement of persons, to immigration into the UK from other EU states—and linked that to increased pressures on wages, housing and public services like health and education.

David Cameron’s promise of a referendum was designed to lure back some UKIP voters and to hold his fractious party together. It also allowed him during the election campaign to distinguish himself from the Labor Party, the Liberal Democrats and other competitors, contrasting his willingness to “ask the British people what they think” with his opponents’ disinclination to do so.

A referendum will push the pro-EU forces to make their case

 Nevertheless, David Cameron’s referendum may be just what the pro-EU forces in Britain need. Labor’s interim leader, Harriet Harman,announced on Sunday that the party will now back a referendum. Contenders to replace Ed Miliband in the Labor leadership have also said they support a referendum.

Euroskeptic Conservatives and UKIP, aided and abetted by supporters in the print media, have successfully ensured that anti-EU opinion is dished up daily to the British electorate.

In contrast, the pro-EU forces have, so far, been in a quiet shambles. The longstanding standard bearer of the pro-EU message, The European Movement, for instance, has been barely visible. The business lobby group British Influence has offered very little beyond a Facebook page that preaches to the converted.

Now that there will be a referendum in 2016 or 2017, the pro-European lobby may finally get into gear.

Pro-EU forces face three major challenges. First, the referendum will be based on the deal Cameron strikes during the renegotiations. Cameron will need to play hardball to win concessions that diminish the role of the EU in certain policy areas and restrict the rights of citizens from other EU states in order to placate Euroskeptic Conservatives.

That may result in a package that some pro-EU forces will not like, but one they will need to support during the referendum.

Second, pro-EU forces will have to convey the intangible and indirect benefits of British membership to ordinary voters. That will be difficult, considering how adept anti-EU forces have been at pointing to tangible disadvantages. Most EU advocates frequently point to the benefits of being inside the EU’s single market, but to a lot of voters, this feels quite abstract and distant.

The pro-EU lobby may wish to learn the lesson of the Scottish independence referendum last year: A “positive change” message can galvanize voters; hence the pro-EU message cannot simply be about the continuation of the status quo laced with fears of a step into the unknown.

The added problem for the pro-EU lobby, however, is that sticking with a Union that is four decades old—the European Union—has less resonance than sticking with a union—the United Kingdom—that is three centuries old.

Third, referendums are unpredictable and hazardous things. While there may be a particular question about the EU on the ballot paper, many voters will cast their ballots answering several different questions, usually linked to their judgment on the government in power.

Current polls suggest 55 percent would vote to remain in the EU (65 percent if asked about remaining “after a successful renegotiation”). But Britons’ attitudes to European integration have fluctuated significantly over the past 40 years and have rarely been top of British voters’ concerns at election time.

Fickle public opinion, the wider lack of relevance to Britons, and the intangibility of what advocates see as the benefits of EU membership offer a three-pronged challenge to the pro-EU lobby in the UK. The fate of the UK’s future in the EU may well depend on its ability to meet that challenge.

Tim Haughton is a political scientist based at the University of Birmingham in the UK.