British politics is hardening. The post-conference opinion polling reported by the Guardian on Monday shows a strengthening in Labour and Tory numbers – both overall, and in terms of determination to vote. To the extent that this is a national snapshot, the nation is falling out of love with the idea of coalition.

This is cheering for Ed Miliband and his decision to take on the energy companies – polarising, provocative "socialism" that the country seems to rather like. But the new dynamic offers Miliband the biggest tactical dilemma of his life. His answer could define British politics for years ahead – and it's got nothing to do with the cost of living, one nation or hard-working people. It's about Europe.

The return to left-right politics, which squeezes the Lib Dems, leaves the Tories still facing their biggest problem: Ukip. Nigel Farage's party is as much an angry annexe of the Conservative family as a new phenomenon. Combined with unreformed constituency boundaries, Ukip voting in 2015 is still likely to rob Cameron of any chance of an overall majority. If Miliband continues to radiate an unnatural sense of calm, Ukip is an important part of the cause.

Yet there is one argument that could yet save Cameron's bacon and deliver a Conservative majority, and it's a simple one, which won't gain full currency until after next year's European elections: "Vote Ukip, get Labour" (because splitting the Tory vote would let Labour in). Rightwing voters are desperate for a European referendum but are bitterly hostile to Miliband; so the Tory message that if you want a referendum, you must vote Conservative will have a lot of resonance. So far Labour is talking about a referendum only if there is a major transfer of powers to Europe. But could that change?

Of all the big tactical plays ahead, this is the big one. You haven't heard much about it yet, but this is a very hot issue wherever key Labour people get together in private. The influential backbencher Tom Watson said recently on television that he might support the Tory rebels' call for an early referendum. He is not alone.

And of course all of this goes way back in Labour history. The ghosts of Barbara Castle and Peter Shore, never mind Hugh Gaitskell (and, for much of his life, Harold Wilson), were never quite exorcised by the New Labour Europhiles. It was, after all, Labour who gave the country the first European referendum.

Imagine the impact if Ed Miliband turned round now and made the pledge. Hard-core rightwing voters would be free to vote with their hearts and Nigel Farage; Cameron would have lost his key line of defence to his right. The Tory party would be further ripped apart as hardcore Europhobes strove to respond by trying to bring the referendum forward. Cameron's careful policy of a nuanced renegotiation followed by a 2017 referendum with all the main parties lined up in favour of staying in, would collapse. Sensation! If Ed thought he was garnering some unexpected media support after his move on electricity prices, this would take him into a whole new league.

Yet this is an Alice in Wonderland story, in which nothing is quite what it seems. Imagine that Labour commits to a referendum, wins the election, and then delivers its promise. At this point, of course, Ed Miliband has neither tried for a renegotiation with Europe nor delivered one. The eurozone is still mired in low growth and high unemployment. With the British right having just lost, and seething, I just don't see how a referendum could be won for the pro-European cause.

So … a new Labour prime minister inherits the aftermath of a no vote. Whatever your views about the EU, the first thing that happens is economic mayhem. Big companies decamp to Frankfurt or Paris. The pound falls. Many of our exporters have no idea what's going to happen to them, and the banks aren't keen to gamble in their favour. This would be an economic storm so big it could overshadow anything else the new government would be trying to do. Ed would be a one-term, unsuccessful prime minister.

Miliband will be all too aware of the risks involved. But he's receiving different advice from people in his party. If Miliband went for a referendum, he would win a short and spectacular tactical victory but at the price of yet more bloody battles between the remaining Blairites and the current Labour leadership.

It isn't worth it. As it happens, and perhaps paradoxically, I'd like to see a European referendum. I've been arguing about this with friends for weeks – but what I don't want to see is Labour rushing into a referendum promise, which would result in us pulling out of Europe shortly after Ed Miliband reaches Downing Street.

So in the end, all this is about confidence and the will to govern. If Miliband genuinely thinks that he has the policies, the character and the disciplined party to win a general election in 2015, then he will not offer an early referendum. It's only if he really doesn't think he's going to make it without a radical reshaping of the political landscape that he's going to be tempted. You want to know what's really going on inside Ed's head? This is the decision you need to watch for.