Photo: Andrew Matthews/PA Archive/PA Images

If late-night chatter in the bars of SW1 about realignment could be converted into votes, centrist politicians would be well on their way to governing Britain. Labour defections? A new party? Vince Cable’s plans for a less tribal politics, with his successor possibly drawn from outside parliament? All are possible; but can any of them break through at Westminster?

Here is the good news for centrists. The democratic world has humbled big incumbent parties in recent years in ways that seldom happened in the half century after the Second World War. Until the new century, the broad pattern in most countries was for centre-right and centre-left parties to dominate the democratic landscape. Today, some have crashed, others shrunk and, in two cases been taken over by total outsiders. If party competition has become so much more fluid, then surely Britain’s political centre, facing one party obsessed by Brexit and another lead by a self-proclaimed Marxist, should be on a roll?

Now for the bad news. Outsiders have broken through in three particular circumstances. None offer succour to Britain’s centrists.

1. A proportional, or semi-proportional, voting system. Nationalist parties from Sweden andGermany to Greece and Italy all established an initial bridgehead in the country’s parliament with well under 20 per cent of the vote. In the UK, first-past-the-post punishes such parties, unless they have geographically concentrated support (such as the SNP). Ukip, the Greens and the Liberal Democrats have all found this to their cost. So, of course, did the short-lived Social Democratic Party in the 1980s. In economists’ terms, first-past-the-post presents a very high barrier to entry. This is great for stability, but terrible for challenging useless incumbents.

2. A two-ballot presidential system where the big traditional parties have fallen out of favour. Emmanuel Macron was especially fortunate. Although he defeated Marine Le Pen massively in the run-off, by 66-34 per cent, he won only 24 per cent in the first round. True, he held a narrow lead; but the four leading candidates all won 20-24 per cent. Given the dynamics of a personality- rather than party-driven election system, and a two-ballot system in which many voters see their first-round vote as a chance to let off steam rather than choose a government, there is a fair chance that Macron would have fallen at the first hurdle in a first-past-the-post parliamentary system. (True, En Marche won a landslide in the subsequent elections to the National Assembly; but by then Macron had already broken through.)

3. A take-over of an established party. Donald Trump and Jeremy Corbyn are the most obvious examples. In the US, the combination of weak national party structures and a drawn-out primary system give a rich, or well-funded, outsider some chance to capture the Democratic or Republican nomination. Here, Corbyn succeeded by exploiting the £3 sign-up rule introduced by Ed Miliband and mobilising left-wing activists who had left, or never joined, the party. In principle, moderates could try to mobilise to sign up even more members. But now that Labour’s membership has climbed to 600,000, that would be a massive undertaking.

So is all lost for Vince Cable’s dreams of a realignment in Britain? Not quite. But the mountain that must be climbed is steep and high. It’s no use aiming 20 or 25 per cent of the vote at the next election. That will yield hundreds of second places, but probably only 30-40 MPs (and even that makes the assumption that a fair number of today’s Labour MPs could hold their seats standing under new colours and against a pro-Corbyn Labour challenger.)

Two conditions need to be met for realignment to work in the next few years. The first is that the Brexit drama produces deep and lasting fissures in both the Labour and Conservative parties, leading to millions of voters willing to desert their usual party allegiance.

The second is that a critical mass of today’s MPs come together to become a significant force in the current parliament. For example, if 130 Labour MPs defected and made common cause with the Liberal Democrats, they could become the official opposition in parliament, relegating the pro-Corbyn Labour rump to third place. Thirty-seven years ago, the SDP failed to attract enough Labour MPs to do this. After a massive early flurry of publicity and wonderful poll ratings, their support declined. Instead of breaking out of their early bridgehead, they were thrown back in the sea.

The lesson from the SDP’s failure is that it is not enough to wish to escape the domination of parliament by an ideological Left and uncompromising Right. Yes, there is in principle a massive gap in the political market. But that seductive notion collides with Lyndon Johnson’s first rule of politics: you must know how to count. The politics of realignment sound attractive; but the arithmetic is daunting.