As Tony Blair might once have put it: immigration, immigration, immigration. David Cameron wants to withdraw child benefit from EU migrants. Labour has come up with a seemingly impossible proposal to restrict people travelling around Europe in search of work. The vice-president of the European commission warns that an overheated debate is in danger of "destroying the future" of the UK, which cleverly has the effect of raising the temperature even more.

Meanwhile, Nigel Farage makes his most significant contribution to British politics yet. On Tuesday, he was on Radio 4's Today programme, echoing something he had already said to Nick Robinson, in the latter's BBC2 programme, The Truth About Immigration: "There are some things that matter more than money." Around a minute into his interview, there it was again: "If you said to me, do you want to see another 5 million people come to Britain, and if that happened we would all be slightly richer, I would say, do you know what? I would rather we were not slightly richer … I do think the social side of this matters more than the pure market economics."

That same morning, I spoke to one of the more switched-on members of the shadow cabinet. "Who was the last front-rank politician you heard confronting the tyranny of the market in that way?" he said. "He's flipped the political conversation. He's hit the switch."

One of the biggest political openings for Farage is easily explained. Both main parties do not think to question the supremacy of supposed growth and the idea that national "prosperity" must be king. Labour politicians have either bought this idea, or dare not depart from it for fear of being called out as anti-business. The Tories, who might once have sounded a more nuanced voice on the basis of conservatism (remember that?), now talk maniacally about the necessity of Britain giving its all in what they call the "global race".

Real people, by contrast, care more about their jobs, where they live, and the fuzzy stuff of security, happiness and a sense of belonging. Politicians affect to understand, but mostly don't. Particularly on the left, to borrow a line from Drew Westen's book The Political Brain, too many people "thrive on policy debates, arguments, statistics, and getting the facts right". A shared fixation with growth figures and national competitiveness is of a piece: more human considerations must be set to one side, lest we fall even further behind the Chinese.

Ukip's appeal has always cut straight through that. Self-evidently, leaving the EU and drastically cutting immigration are not ideas grounded in economics: they are cultural and emotional, even if the party seemed bashful about admitting as much, due to its Thatcherite values (symbolised by Farage's background as a commodities trader). In that sense, up until this week, it was not quite right to characterise them as populists: they seemed too in thrall to the free market. Now, though, Farage seems to be on manoeuvres.

His latest pronouncement highlights a huge hole in our politics – indeed, you could apply it to no end of issues, some of which would not be to Ukip's taste. Would you like Britain to bin all the "green crap" so that GDP might go up a bit? Cue the Farage gambit: "Do you know what? On that basis, I would rather we were not slightly richer." What do you think of the countryside being fracked? Ditto. How about a high-speed rail line ploughing through rural England? The same. Aside from Ukip, the only other party I can imagine sounding such notes is the Greens, which says something about the extent to which Farage threatens to have this territory largely to himself.

Those who believe in the free movement of people should now understand what they have to do: stop yakking on about such abstract matters as migrants' net contributions to the economy, and start to talk about what people from overseas bring to the communities where they live (while, if at all possible, also acknowledging that large-scale immigration is unavoidably disruptive and disorientating, something too often missed). As for Ukip, we should also understand that there is now officially much more to its appeal than all that useless talk about Enoch Powell.

That doesn't mean much of their campaigning isn't ugly, or that their chief effect on the Conservative party hasn't been to push it towards a new kind of nastiness. But as my shadow cabinet contact said, a switch has been hit, by a party leader whose crafty sense of what politics is lacking gets ever sharper – and the consequences could be explosive.