THE opposition Labour Party is about to inflict grave damage on Britain. If it picks Jeremy Corbyn, a veteran far-left MP, as leader on September 12th, Labour will consign itself to the wilderness. Worse, by wrecking opposition to the governing Tories, Mr Corbyn will leave Britain open to bad government.

The sudden vogue for populist leftists like Mr Corbyn echoes the earlier rise of parties such as Syriza in Greece and Podemos in Spain. Similar enthused crowds have been greeting another grizzled old socialist, Bernie Sanders, in America (see Lexington). All of them have energised new, mainly young supporters who fret about globalisation and inequality.

Yet even in such dubious company Mr Corbyn stands out as a throwback. For him no policy is too dog-eared, no intellectual dead-end too futile. Public spending? Yes, please. Higher taxes? Soak the capitalists and the landlords. State ownership? Nationalise the railways and utilities, get the private sector out of public services and reopen the coal mines. If that were the secret of prosperity, Britain would never have fallen apart in the 1970s and Tony Blair would not have won three elections at the head of a modernised centre-left Labour Party.

No prizes for concluding that Mr Corbyn would not get The Economist’s vote. He is stridently anti-American and anti-Israel—though he is a “friend” of Hamas and Venezuela. To him, Britain’s nuclear weapons are evil and always were. NATO is a warmongers’ plot to enrich the military-industrial complex. The European Union risks being a Trojan horse for liberalism.

Our worry is not that Mr Corbyn would win power. Labour grandees—including Mr Blair and Gordon Brown, the party’s most recent prime minister—have lined up to declare, rightly, that he is unelectable. Even under its former leader, Ed Miliband, voters found Labour too left wing. Our worry is that Mr Corbyn will do immense damage even as opposition leader.

First to Labour itself. He does not have the loyalty of Labour MPs, who know him best (see article). He barely scraped together the 35 MP nominations he needed to stand—some backed him only because they thought he would broaden the debate (they got what they wished). As leader, he will control the levers of party power; the hard left and the unions will help him use these for their own ends. Even if Mr Corbyn does not last, his successor will struggle to clean up the mess.

Second, he will harm the Conservatives. Many Tories are cheered by the prospect of Mr Corbyn as Labour leader. Just as in the 1980s, they say, his loony-left policies would win them elections. In this they are undoubtedly right. But they are still wrong to welcome his prospective leadership.

Truth and faction

Good government needs a coherent opposition to scrutinise it and hold it to account. Under Mr Corbyn, hard-left policies, his lack of support among MPs and his own record as a serial party rebel will make this impossible. A shambolic Labour Party, and a rump Liberal Democrat Party that has also drifted left, will leave the Scottish nationalists as the most potent opposition to David Cameron’s government.

The Conservative Party is itself a coalition—of English nationalists, libertarians and “one-nation” Tories. Without the discipline of strong opposition, these factions tend to fight each other—and Mr Cameron. Having only a slender majority, the government is vulnerable in difficult forthcoming votes, such as those on air strikes in Syria or airport expansion in London. The referendum on Britain’s EU membership will become more unpredictable. Britain needs an opposition that lives in the real world and a united, focused government. With Mr Corbyn as Labour leader, it risks having neither.