Incumbency and the sophomore surge: why these two effects will matter for the Lib Dems in 2015

by Stephen Tall on August 4, 2014

Lib Dems winning hereWith the Lib Dem vote at least halved in the polls since the last general election, there’s been much focus within the party on what’s known as the ‘incumbency effect’ – the personal vote that benefits Lib Dem MPs. This typically boosts Lib Dems by 8%, compared to 1-2% for Tories and 1.5-2.5% for Labour MPs.

It’s this effect which, Lib Dems hope, will enable the party to buck the national trend at the next election. It is, however, limited to those seats where the current MP will re-stand: currently nine Lib Dem MPs, 16% of our 57 MPs, have announced their retirement. In those nine seats the party will lose that incumbency effect – in effect, starting c.8% behind its position in 2010.

There is another electoral effect – the ‘sophomore surge’ – which may well also have an impact: this is the additional polling bounce a first-time MP gets when up for re-election. The Lib Dems have 11 first-term MPs (if you include Eastleigh by-election winner Mike Thornton), 19% of our 57 MPs, who would expect to see at least some degree of sophomore boost to their positions.

The effects of both incumbency and the sophomore surge have been calculated by Tim Smith, a PhD student at the University of Nottingham, on the Ballots and Bullets blog.

Let’s start with the bad news: “even before there is any swing against the [Lib Dems], they are four seats down, purely due to retirements.” Those four seats are Berwick-upon-Tweed, Mid Dorset & Poole North and Somerton & Frome (all with Tories in second place) and Brent Central (Labour). Sir Malcolm Bruce’s Gordon constituency is also seen as risky.

But there is better news:

The effect of having ten first term incumbents is that some of the party’s seats will become easier to defend. The distribution is also important since seven of the ten seats are in constituencies with majorities under 5% of the vote over the nearest opposition party. This will mean that if there is a swing against the party, the number of seats beyond those already expected to go due to retirement effects might well be lower than would otherwise have been expected. The adjusted majority of the 20th safest seat the Lib Dems have on paper is 7.34%. Taking incumbency into account the 20th safest (including the 4 that would be lost) would be larger at 8.28%. Therefore with a theoretical swing of approximately 4% against them, the Liberal Democrats end up having more seats once incumbency is taken into account than they would on paper.

Liberal Democrat MPs have much larger incumbency advantages than those of other parties and this is likely to have important effects at the next election. The balance of the effects improves the worse the party does: a tiny swing towards the Liberal Democrats might see them losing seats, all other things being equal, whilst a large swing against them might be partially mitigated by having so many first term incumbents.

* Stephen Tall is Co-Editor of Liberal Democrat Voice, and editor of the 2013 publication, The Coalition and Beyond: Liberal Reforms for the Decade Ahead. He is also a Research Associate for the liberal think-tank CentreForum and writes at his own site, The Collected Stephen Tall.