This newspaper was unconvinced by David Cameron's promise in his speech in January of a referendum on Britain's membership of the European Union. We feared that he might be trying to repeat Harold Wilson's manoeuvre of 1974-75, but that this time it might lead to the country leaving, rather than confirming its membership of, the EU. We said that, although a referendum at some point would probably be necessary and democratic, announcing it so far in advance risked four years of business uncertainty. And we were concerned that, by trying to buy off the Ukip threat on his right flank, the Prime Minister had succeeded only in making respectable the notion that withdrawal would end all our irritations with the EU.

After the Eastleigh by-election, and perhaps unexpectedly, The Independent on Sunday is prepared to grant Mr Cameron a little more of the benefit of the doubt. Nigel Farage, the Ukip leader, may well have been cock-a-hoop at his party's pushing the Conservatives into third place. The strong showing for Ukip has prompted us to pay the party the compliment of taking its policies, personnel and backers a little more seriously. What emerges, blinking, into this daylight is unattractive. It may not have been wise for Mr Cameron to describe the party once as "fruitcakes, loonies and closet racists", but the policies of the party are driven not by Euroscepticism so much as hostility to immigration. Mr Farage's critics accuse him of the politics of the dog whistle. The foghorn, more like.

Yet the important point about the by-election is that Mike Thornton, the Liberal Democrat, won it. As John Rentoul points out today, if Ukip cannot win there at a time like this, it must be doubted whether it can ever do more than siphon votes from the one mainstream party that could deliver the EU referendum that its supporters want.

It is worth wondering, furthermore, what might have happened in Eastleigh if Mr Cameron had not made his promise of a referendum. Then the Conservatives might have been in real trouble, and, interesting though that might have been, it would hardly have been in the national interest to have Mr Farage more rampant than he already is.

This is not the only reason for thinking that the Prime Minister's acceptance of a referendum might have more to commend it than we thought, even if it were actuated by party advantage. Since his speech, polls have recorded a marked shift in public opinion in favour of staying in the EU, even if it is not yet supported by a majority, which was the position before the eurozone crisis began in 2010.

If there is a case for renewing the democratic mandate for British membership, which this newspaper believes that there will be when the EU and the eurozone have resolved the present crisis, political leaders should say so now. The position adopted by Ed Miliband, the Labour leader, is unsatisfactory. He told the Commons in January, "We don't want an in-out referendum," although it may be that he left out the word "now" by mistake.

But if the principle of democratic consent is right, as it is, Mr Miliband should follow Mr Cameron in saying so. It allows the debate to move on to the substance rather than the mechanism. Is it in Britain's interest to remain in the EU? We believe that it is, and that the prospect of a referendum allows us to expose the backward-looking arguments of Ukip and its supporters without distraction.