Corbyn wants the EU to gain more powers to protect workers, while Cameron wants Brussels to have less power to tell British businesses what to do.
The prime minister will claim that whatever concessions he gets from Brussels about reducing EU influence on British business will be sufficient to justify staying in. Corbyn has charged that EU concessions given to a Conservative government would be ‘damaging changes’ because they would favour business.
To convince referendum voters that his pro-worker changes can be achieved, Corbyn will have to sell them two connected promises: that he can become prime minister in 2020 (and so be in a position to push for change in Europe), and that he will be able to convince a majority of other EU member states to endorse left-wing reforms (that most do not currently accept).
However, the bigger problem is the EU referendum ballot will not actually offer voters the chance to vote for the reforms that Corbyn wants. People must make a straight ‘in’ or ‘out’ choice about the EU as it is, and so his qualified endorsement of the EU will confuse Labour supporters.
Their own positions on the EU are far from clear. The British Election Study found in May that while Labour voters are almost 3 to 1 in favour of the EU, less than half are definitely committed to voting to stay in the EU. The combined effect of a Conservative prime minister campaigning for continued membership, and Corbyn being of two minds about the EU, is likely to eat into this contingent support. Moreover, uncertain voters are less likely to turn out to vote than certain voters; among Labour supporters three-fifths are not certain of their EU views.
His difficulty is furthered by two aspects of the party’s internal dynamics. If Corbyn decides to endorse staying in Europe on terms he dislikes, he will invite a split with left-wing union leaders – who value the EU insofar as it gives workers rights that a British Conservative government would not provide.
During the Labour leadership campaign unions demonstrated their ability to get their members out to vote, and the British Election Study finds that union members have been less likely to favour the EU than Labour Party members.
Conversely, if he endorses leaving the EU, then he will invite a split with senior shadow cabinet ministers who want Britain to stay in the EU – starting with Hilary Benn, the shadow foreign secretary.
To resolve tensions within the Labour Party and within himself, Corbyn could accept that the Labour Party organisation remains neutral during the referendum campaign. Pro-Europe MPs and unions would be free to campaign to stay in and left-wing unions and MPs to get out.
However, Corbyn’s continuing ambiguity will be bad news for those wanting to stay in Europe, because Labour supporters have been the biggest bloc of partisans in favour of the EU. While Liberal Democrat, Scottish National and Green voters are even more pro-EU, the Labour vote will have a much bigger impact on the referendum outcome, because Labour has twice as many voters as the other three parties combined.
Professor Richard Rose is director of the Centre for the Study of Public Policy at the University of Strathclyde (http://www.cspp.strath.ac.uk). This blog was written as a grantholder of the ESRC UK in a Changing Europe Initiative.