Could a new centrist party plug the gap in British politics?

Jeremy Corbyn and Theresa May.

Could a new party break the Labour-Conservative grip on the House of Commons? Such a party is mooted in this week’s Observer, with a reported £50m available to get it going.

On the face of it, the precedents are not encouraging. The obvious example is the Social Democratic party, formed in 1981 as an alternative to Michael Foot’s Labour party and Margaret Thatcher’s Conservatives. It sought to “break the mould” of British politics but failed to break through, winning only 23 seats in the 1983 general election. Subsequent attempts to break through, such as James Goldsmith’s well-funded Referendum party in 1997 and Ukip in 2015, also stumbled and quickly fell away.

However, it is lazy to assume that the mould can never be broken. Precedents are not iron laws. To make a proper assessment of the chances of a new party, we need to understand the reasons why past efforts failed, and then judge whether those forces could be overcome this time.

The table below sets out the nature of the headwinds that confront any new party. It provides examples of how elections to the House of Commons have played out for three kinds of party with less than one-third of the vote: those that attract broadly similar levels of support in different areas; those that have a similar level of appeal in most places, but manage to build up extra support in particular constituencies; and those that are particularly popular in some areas, and unpopular in others.

The final column is the one that matters most: it shows how well each party converts votes into seats. The figure of 100 means that the proportion of seats exactly matches the proportion of votes. The lower the number, the worse the conversion rate, the higher the number, the better the conversion rate.

Peter Kellner chart

The message is clear. As section 1 of the table shows, the Liberal-SDP Alliance’s failure in 1983 was a straightforward product of our first-past-the-post voting system. They won more than 6m votes, and did respectably well almost everywhere. They racked up as many as 313 second places – but won only 23 seats. Other parties with evenly spread support have done as badly, or even worse: only nine years earlier, the Liberals had won 20% of the vote but just 14 seats.

As recently as 2015, Labour, which had dominated Scottish politics in most seats north of the border, lost ground almost everywhere. From winning almost 70% of the seats with 42% of the vote in 2010, it crashed to 24% of the vote and just one seat in 2015.

Section 2 shows that parties with a semi-even distribution of votes do somewhat better, but still fall well below 100 in the vote-seat conversion rate. Thus in 1997, the Lib Dem vote was actually down on 1992 (from 18% to 17%), but it more than doubled its MPs, partly because it did exceptionally well where its candidate was well-placed to defeat the incumbent Conservative MP. This extra support, concentrated in a few dozen seats, lifted its vote-seat conversion rate from 17% to 41%. In 2010, the election that led to the Conservative-Lib Dem coalition, its conversion rate was much the same: 38%.

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