New research on the attitudes of party members across the UK points to the challenge before David Cameron if he is to achieve a “yes” vote on the EU referendum without splitting his party badly.

Cameron wants to re-negotiate the terms of UK membership of the EU, and then obtain a majority in the referendum for continued membership on those terms.

A survey of party members that we conducted shortly after the general election reveals both good news and bad news for the prime minister. The good news is that just 15% of Conservative Party members would vote for the UK to leave the EU – regardless of whatever reforms Cameron manages to negotiate. Nearly two-thirds (63%) say their decision would depend on the outcome of the negotiations; one in five say they would vote to stay in the EU no matter what.

So, a clear majority of the Tory grassroots would vote to stay in, or are at least persuadable, depending on the outcome of negotiations. That’s all encouraging for the prime minister. The problem is, what happens if he cannot persuade his members that he has achieved reasonable terms for continued membership?

Members on the edge

There are two clear dangers in this for the Conservatives. The first is that they will split and some may ultimately quit the party. Some may even defect to UKIP, the party with the most uncompromisingly hostile position towards British membership of the EU.

This is not an insignificant threat to the Conservatives. In a 2013 survey of Tory members, we found those who were most likely to foresee the possibility of their eventual defection to UKIP were also the most likely to insist on leaving the EU, even if the government were able to negotiate a significant repatriation of sovereign powers to the UK.

Clearly, EU withdrawal is a touchstone of dissent for rebellious Tories. It is certainly not far-fetched to imagine a scenario in which a Conservative Party badly divided on the referendum question eventually haemorrhages members, councillors, MPs and voters to UKIP (or less dramatically, perhaps, to apathetic non-participation in party politics).

The second danger lies in the loss of a valuable organisational resource. Notwithstanding the clear financial advantage the Conservatives enjoyed over all other parties during the election campaign, research suggests that the local campaign efforts of volunteer activists still make a significant difference to electoral returns at the constituency level.

For the Conservatives, the members most likely to have put in hard graft for their candidates during the election campaign are also among those most likely to favour EU withdrawal. While 13% of those admitting to doing no work for the party said they would vote for EU withdrawal whatever the circumstances, 15% of those working up to 30 hours per month said they would, while 25% of those doing more than 30 hours per month said they would. The numbers falling into the last of these categories is only small (about 5% of the membership), but they include some of the most important of the party’s activist hardcore.

Beyond the Tories

Of course, it is not only the Conservatives for whom the referendum carries risk. Recent media reports point to various concerns among Labour strategists too. If they end up campaigning with Cameron (presumably) for continued membership, will they somehow be tarnished in the eyes of their usual supporters, as they appear to have been in the Scottish independence referendum? And might they further alienate elements of their core working class supporters who appear tempted by UKIP?

On the other hand, to oppose EU membership would surely further damage the party in Scotland, where support for the EU is generally greater than in England.

At least Labour can take solace in the very clear steer provided by its own membership. Few (only 5%) favour British withdrawal from the EU regardless of any renegotiations. Indeed – apart from UKIP, whose membership is implacably opposed to continued UK membership under any circumstances – the other main party memberships take a remarkably consistent line on the issue.

Attitudes of party members to the EU referendum / Paul Webb & Tim Bale

As many as 85% of Labour members declare that they will vote to stay in regardless of the outcome of any negotiations, as do 86% of Liberal Democrats, 82% of Greens and 81% of SNP members.

The greatest underlying threat to party unity from the referendum, then, would seem to lie with the Conservatives. It will take consummate political skill on the part of the leadership to navigate the waters of the referendum campaign without being holed below the waterline.

Paul Webb is Professor of Politics at University of Sussex.
Tim Bale is Professor of Politics at Queen Mary University of London.