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Predicting Liberal Democrat seats at the election: incumbency, swing, tactical voting and unknowns

by Timothy Smith

Among all forms of mistake, prophecy is the most gratuitous.” – George Eliot – Middlemarch

It has always been difficult to predict the likely numbers of Liberal Democrat MPs ahead of an election, and with the exception of 2010, predictions based on exit polls have generally been at least 10% off beam, sometimes much more.  At the 2010 election, the Liberal Democrats lost a net five seats despite increasing their vote share from 22 to 23%.  The results saw seats moving in both directions with the party gaining and losing seats from both the Conservative and Labour parties.  The big drop in support that the Liberal Democrats have suffered since joining the coalition in 2010 means that it is next to certain that they will lose a large number of seats at this election.  This post discusses the factors that will decide how many of the 57 seats the party won in 2010 it can hold.


This author has already written about incumbency factors here and here and discussed the effects of Liberal Democrat retirements in a post here. However it would be a mistake to assume that incumbency will be a ‘get out of jail free’ card for the Lib Dems, should things go badly on election night.   Research by this author and others has repeatedly shown that an MP’s incumbency accrues after one term in office, the sophomore surge, but does not increase with each additional term.  Whilst the first term MPs, nine of whom are re-standing, are likely to see a first time surge, all are in vulnerable seats and could be swept away regardless.

There are also a larger number of Lib Dem retirements than usual, with 10 of their MPs standing down, and another is fighting his party as an independent.  In these constituencies, the party is likely to do badly as the accrued incumbency advantage of the previous member is lost in addition to the likely adverse swing.  There is additional uncertainty this time as to whether the size of the observed incumbency advantage gained by Lib Dem MPs could change given the unusual change in the party’s overall support..  It is possible that the party’s unpopularity could inhibit supporters of other parties from responding positively to a Lib Dem MP’s work in the constituency and lending their vote.  Equally, it is possible that the value of incumbency could increase if voters decide that their Lib Dem MP is not responsible for the unpopularity of the party nationally.


Since David Butler and Robert McKenzie introduced the Swingometer on the BBC’s 1955 election night coverage, it is has been popular to use the concept of uniform national swing (“UNS”) to illustrate the extent to which shifts in votes leads to seats changing hands.  UNS did not work at all well in predicting changes in Lib Dem seats at the 2010 election.  Had the observed swing of 1.4% from Lib Dem to Conservative been uniform across the country it would have meant eight seats moving from Lib Dem to Conservative.   Instead, there was a net gain of nine seats by the Conservatives, but only three of the eight that should have gone, did.

If the polls as of mid-April are right, the UNS model will be even less helpful for this election.  Assuming a drop in vote share in Great Britain from 23.5% to 8.5%, UNS would predict a 15% drop in all constituencies, resulting in an impossible negative Lib Dem vote in 131 seats, including a minus 9.97% in Glasgow East.   Other methodologies such as ratio or transition models explained here, may have more success, but there has not been such a large decline in the centre-party’s vote in a single election in the post-War period.

Tactical Voting

Liberal Democrats have relied on tactical voting by partisans of other parties who vote for them to keep out a different party they dislike.  As the centre party they have benefitted from the fact that in seats where they win, or are in a close second place, a proportion of supporters of the weaker of the two major parties lends them their votes in order to keep the other major party out.  The problem for existing Lib Dem MPs is that at this election they may find that some of this tactical vote could unwind.

Data from early results of the 2015 British Election Study (BES) shows that there has been a major deterioration in voter attitudes towards the Lib Dems since 2010.  Voters in both surveys were asked to rate the parties on a scale of 0 to 10, with 0 being strongly disliked, and 10 being strongly like.   In 2010, the Lib Dems were viewed relatively favourably amongst people who had voted for the two major parties, receiving a mean rating of just over 5 from Labour voters and 4.3 from Conservatives.   The graph below shows the deterioration seen in the early BES data for this election.  The Lib Dem rating is now barely 2 amongst Labour supporters, and has also fallen amongst Conservatives to just under 3.  Labour partisans now barely distinguish between the Liberal Democrats and Conservatives, with the mean difference between the two at just 0.4-pts.   This is dangerous for the Lib Dems since the Conservatives are the runners up in the majority of their seats, and so they rely more on the tactical votes of otherwise Labour voters.



Finally, at any election there are always surprises, and with the breakdown of national trends, these surprises have become more frequent and potent.  Examples in the 2005-10 cycle included the MP’s expenses scandal, the Clegg-mania after the TV debates, and the unusual number of local-issue led results such as the Greens winning Brighton or Lembit Opik losing Montgomery. There are now only three weeks to go until polling day and so time is running out for any surprise game-changers.  However, the nature of the location of the Lib Dem seats in far flung places, literally Lands End to John ‘O’Groats, increases the odds of undetected local idiosyncrasies affecting results.  The above analysis suggests that the Liberal Democrats are going to have a very bad election indeed, perhaps a lot worse than the 25 – 30 seats that the betting markets are predicting.

Tim Smith is a part time Politics PhD student at Nottingham University

Image Credit: Wikipedia Commons

Published inBritish PoliticsGeneral Election 2015Liberal DemocratsPolitics


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