David Cameron has crossed the Rubicon. There is no going back. By proposing to limit free movement of labour from the EU he has planted himself on the side of the outs, as José Manuel Barroso made crystal clear in his Chatham House speech on Monday. The other 27 nations will never agree: if limiting national insurance numbers for EU workers is Cameron’s new red line then he has joined the Ukip wing of his party, who won’t let him renege.

“I will go to Brussels, I will not take no for an answer … when it comes to free movement,” he told his conference, and now that’s confirmed. In a riposte to Barroso’s plain statement of the facts on EU law, No 10 rudely warned him that he “should be under no illusion that the status quo is not acceptable to the UK”. Cameron went one worse and told him “who is the boss” on immigration.

There is no effective difference between Ukip and the Conservatives, both heading for the exit – except, as Nigel Farage says, Cameron “is deceiving the British public” with nonexistent options, breeding more political cynicism.

Cameron is no Caesar. He has been dragged backwards across this Rubicon by his enemies, in the long lurch away from Europe that began when he used an anti-EU ploy to secure the Tory leadership. Leadership? Not really, he’s been trailing after the Europhobes ever since, trying to keep up. Every step since then he has conceded British interests to appease the unappeasables. He won with a promise to withdraw the Tories from the European People’s party, outraging natural conservative allies such as Angela Merkel.

That bartering of British influence was the telling moment: Cameron would put his political interest ahead of his country’s in ways that would surely shock previous Tory prime ministers. Would any of them deliberately risk the nation’s pivotal international relationship just to swing an embarrassing byelection in Rochester? Cameron’s stated view is that Britain is better off inside the EU – but not, it seems, if it puts him to much inconvenience.

Until now Cameron has wisely refused to spell out negotiating terms for reforms he would seek before a 2017 referendum. He might have held to that if he made the broad case for Europe, rebutting his backbenchers. But as his anti-EU rhetoric heated up, the Europhobes demanded red lines so blood-red they would be impossible to achieve, guaranteeing British exit. By definition, nothing Cameron suggested that might be agreeable to the other EU 27 would be enough for his outists or for Ukip. Only impossible demands would be enough – a bizarrely Trotskyist revolutionary tactic from the likes of Jacob Rees-Mogg and John Redwood.

That’s the trap his enemies have cleverly pushed him into. It wasn’t hard, as they saw he never had the nerve or conviction to stand and fight. They saw that character flaw, and he’s despised for it on all sides of his party. His strategy against Ukip has been a disaster: first he ignored them, then he insulted them, now he imitates them. He never tried making a robust EU case against them.

Barroso, safe in the last days of his term, laid bare his contempt for the Cameron government’s Euro-madness, spelling out the consequences of British withdrawal: banks and businesses have told him they would leave, along with car manufacturers and financial services. The business secretary, Vince Cable, hastened to give the same warning, as he visited a Ford factory making the sort of new investment that would stop. Out of the EU could mean Scotland departing, losing the UK’s UN seat and US indifference to us as an island of irrelevance. In exchange for what?

Barroso’s warning is a reminder of how decadent Conservative anti-EU obsession looks to the rest of the world. The eurozone, our biggest trading partner, teeters on the verge of another nervous breakdown. British households still struggle with falling incomes. Russia is flexing its muscles alarmingly on EU borders. Ebola in west Africa needs urgent assistance. Global warming gallops on, hardly checked.

Some Ebola of the brain has seized the ruling party as Cameron now “bangs on” about Europe. The May election is set to be an in/out decision in itself: if Cameron were to win it looks less likely he would take the pro-in side in a referendum.

Labour has been nervously muted on the EU and reluctant to take on Ukip. But at last they realise there is no option but a vigorous fightback. Ed Miliband’s mocked conference speech was strong on Europe, calling Cameron’s “pandering” to Ukip the reason why he’s not fit to be prime minister. “Let me say it plainly: our future lies inside not outside the European Union.” Reforms are needed but “do we reform Europe by building alliances or by burning alliances?”. Cameron has no chance of negotiating with “one hand on the exit door”. “The biggest threat to our prosperity is now the Conservative party.” Arguing for staying in, even if it’s unpopular, is Labour’s best proof of economic responsibility, as the Tories swivel off the eurorails. Many Labour MPs were heartened when Miliband told the parliamentary Labour party last week that he would now tackle Ukip head on.

Immigration rides near the top in public concerns – and it frightens all parties. Labour can promise reforms on benefits for migrants and on new accession states – but that won’t satisfy the Ukip-minded. Douglas Alexander, the shadow foreign secretary, fresh from the Scottish referendum, says the winning message must be the same: promise achievable reform from the safety of staying within the union.

As a pro-EU party, Labour can take the credit for honesty – unlike Cameron. Admit life is full of difficult trade-offs: leaving the EU would harm the country far more than taking in migrants who are young and productive. But they could say that money brought in by migration – worth several billion a year – should be pledged for those communities who most need new homes and schools, the poorest whose anger has been turned against newcomers.

Of course that won’t assuage the rage of Faragists who hate foreign languages spoken on trains. But it may be grudgingly accepted as common sense by most. After all, with few making the pro-EU case loudly enough, polls still find more people wanting to stay in. Time for Labour to bang the drum louder.