The government’s bill for stopping boats will stop no boats. But its job is already done. The mission was accomplished on Monday night when opposition MPs voted against the proposed law at second reading in the Commons. There are more legislative hurdles to clear, yet Suella Braverman was exultant. Keir Starmer’s party, the home secretary said, had proved that it wanted “open borders and unlimited migration”. That isn’t even a distortion of Labour policy. It is a falsehood animated by a fantasy of the party Braverman and her colleagues want to fight at the next election. For as long as there is an argument over the bill, the Tories will present themselves as the last defence against a migrant armada and its accomplices. The fifth columnists were named in Monday’s debate by Scott Benton, MP for Blackpool South: “lefty lawyers and celebrity do-gooders”. Which of those is Theresa May? She is famous, but not in the field of liberal pieties around migration. And yet, even May, author of the home office “hostile environment” policy, thinks Braverman has gone too far. May is exercised over clauses in the bill that undermine the 2015 Modern Slavery Act, which was the compassionate part of her legacy. The new law would deny anti-slavery protections to anyone who entered the country illegally, which is the normal route for those who are smuggled by traffickers. It takes a Kafkaesque flourish of spite to withdraw help from desperate people on the grounds that they committed the crime of which they are the victim. That is one of many sinister absurdities in a policy made for use on a campaign leaflet. Pledging to stop the boats is considerably easier than stopping the boats. There is no evidence of any deterrent effect from previous efforts to strip charity out of Britain’s asylum system. There is also no mechanism for dealing with all the people who will have their claims to sanctuary automatically and irreversibly invalidated (in breach of the UK’s commitments under international conventions on refugee rights). They will end up in legal limbo, pending removal to Rwanda, or some as yet unidentified country. Without mass deportations, the bill condemns asylum seekers to internment or destitution. The cost will rise; the boats will still come. But for Conservative MPs that is a problem for someone else, or for their future selves, on the other side of an election. Only a handful of Tories, grandees who are beyond ministerial ambition and dissidents with plans to quit parliament, recognise that this is a bad way to make law. Bad in the moral sense that thinks democracies should not be wantonly cruel, and bad in the constitutional sense that parliament is supposed to do more than scrawl campaign slogans on to the statute book so it can say the opposition wants to erase them. This isn’t new. Governments have often passed ill-conceived laws with unforeseen consequences because they lacked the time or imagination to draft better ones. The Commons is a theatre where success is measured in performance, not quibbling over the script. Parliamentary votes have been called before to force the opposition (or sometimes the government) to cross some line that puts them on the wrong side of public opinion. But the illegal immigration bill represents a new stage of constitutional degradation. It takes the most dysfunctional elements of Westminster process and applies them as the deliberate instrument of government policy. A number of factors have come together to make this possible, but Brexit is the catalyst. Years of legislative trench warfare over implementation of the referendum result bred fear of parliamentary scrutiny in the hardline Eurosceptics. That faction is as scornful of Britain’s obligations under international human rights law as it was of the case for frictionless trade with the EU, smelling subordination to foreigners in both spheres. They see due process as a trap and the Commons, even with a Tory majority, as a place where the will of the people can be ambushed and emasculated by unpatriotic liberals. In that view, Boris Johnson was right to attempt an illegal suspension of parliament in September 2019 because MPs wouldn’t do his bidding. He was right, too, to pass a Brexit deal that he had no intention of honouring, because the terms of the treaty were unimportant compared to the principle of emancipation from Brussels. Brexit demanded a separation in the Conservative mind between ideological ambition and practical government. To prioritise the latter was an offence against the former. The two spheres move further apart the clearer it gets that none of the purported benefits of leaving the EU is real. The election of Liz Truss as Tory leader last summer expressed the party’s determination not to have the bubble of its fantasies pricked. Rishi Sunak was summoned to the job, in deference to gravity, because the bubble burst. But the years of wilful flight from seriousness have had a lasting effect. There is a glibness baked into British politics; a reflex recoil from hard questions. Public debate struggles to escape the gravitational pull of Twitter-led culture wars, a safe space for people who find technical policy boring but like to argue about politics in grandiose, moralising terms. By that mechanism, the future of Britain’s asylum policy ended up subordinate to a debate about Gary Lineker’s tweets. The fitness of BBC social media guidelines has had more scrutiny than a bill awarding the home secretary enhanced powers to detain children. On Monday night, parliament considered the question of whether Britain was still a country that recognised the universality of rights that were codified after the second world war. Most Tory MPs decided it was not, although few who supported the bill had the courage to put it in those terms. They say they are stopping the boats. Some of them probably believe it too. They think that saying it must be done is the same as getting it done. When it turns out that it can’t be done, not their way, and the consequence is chaos and misery, they will blame the people who warned them it would be so. That has been the cycle of British politics since 2016. There was a moment, when Sunak became prime minister, that a different path was visible. He styled himself as the man to restore responsibility and sobriety to government, which was a low bar to clear after the Truss debacle. He restored grownup diplomacy in relations with the EU and was rewarded with a deal on Northern Ireland. But he also reappointed Braverman to the Home Office and hitched his electoral prospects to a migration policy built to fail with flamboyant cruelty. He has made “stop the boats” a test of his credibility, at which point he decided he was not serious about being serious. Rafael Behr is a Guardian columnist