Opinion polls in Britain show rising anti-EU sentiment. Prime minister David Cameron is promising a speech later this month to lay out his thinking on a new treaty, with a referendum to follow. He thinks Britain should stay in the Union, but says exit is imaginable in certain circumstances.

That posture is causing predictable dismay among British euro-skeptics, who just want out, and also among pro-EU types, who think it's insane even to raise the subject of bringing powers back from Brussels. We'll see what he says in the speech, but Cameron's basic approach seems right to me. Whether it makes sense for Britain to stay depends on where the EU is heading--on the terms of the partnership. If Britain and the others move sufficiently far apart on what the EU should be, then it would better for everybody if Britain negotiated a friendly exit. This possibility shouldn't be posed (or interpreted) as a threat, or an attempt at blackmail. And it certainly shouldn't be regarded as "unthinkable". On the contrary, it should be examined very carefully.

I chuckled to read about the Obama administration's thinking on the point. The US wants Britain to remain in the EU, according to an emissary in London, and here's why:

"We benefit when the EU is unified, speaking with a single voice, and focused on our shared interests around the world and in Europe," Philip Gordon said during a visit to London, adding: "We want to see a strong British voice in that European Union. That is in the American interest."

Gordon stressed that it was it was up to Britain to determine its European role but, in what appeared to be a clear reference to attempts to renegotiate UK membership with the EU, he said: "It would be fair to say that every hour at an EU summit spent debating the institutional makeup of the European Union is one less hour spent talking about how we can solve our common challenges of jobs, growth, and international peace around the world."

Washington lectures London on the foolishness of debating process instead of getting to grips with real problems? It wants Europe to be less obsessed with its own internal wrangling and more outward-looking? Priceless. Not to mention the fact that the United States would never consent, would not for one nano-second think of consenting, to a fraction of the pooling of sovereignty that Britain and its EU partners have already undertaken, let alone the further pooling now contemplated.

I think Britain should call on the US and its partners to start recasting NAFTA as a full monetary and political union. Be visionary, for heaven's sake. Put the Federal Reserve in Mexico City as a measure of good faith. Europe would benefit if North America were unified, speaking with a single voice, and focused on shared interests around the world and in the region. Also, it would be so much easier for Europe to deal with one government in North America instead of three.

What's that? It's not so simple? No, it isn't.