Jon Day at n+1:
It began as an internal matter of party discipline. The offer of a referendum was a strategic decision made by the Conservative Party in the run up to last year’s general election. It was offered both as Prime Minister David Cameron’s concession to the eurosceptic wing of his own party—which had been hammering him on the issue of EU membership from the shires of middle England for years—and as a way of shoring up his nationalist credentials against the upstart band of blazer-wearing, spittle-spewing paranoiacs who call themselves the UK Independence Party.
Though they have one MP in the Commons (their leader Nigel Farage sits prettily in the European Parliament even as he rails against it) UKIP are more of a single-issue pressure group than a real political party. Nevertheless they have proved depressingly effective at whipping up anti-outsider sentiment: against Romanians and Bulgarians, against Turkey joining the EU, against Syrian refugees. It seemed as though they might well dilute the vote for the Tories—as well as for Labour, in some constituencies—during the 2015 election, so that giving way on the question of an EU referendum made some sense for the Conservatives. Both major parties felt comfortable, if not compelled, to make immigration a campaigning issue.