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Lib Dem incumbency advantage persists but fails to prevent disaster

By Tim Smith

As this author warned here, some of the assumptions that incumbency advantage would prevent a poor result for the Liberal Democrats at the election were flawed.  As the post suggested, the party did indeed do far worse than the projection that they would hold onto at least 25 seats. As David Steel said, decades of progress were reversed with the party finishing up with just eight seats, the lowest since 1970, and it was arguably its worst result since 1959 in terms of share of the vote.  The party lost all of its seats in its strongest English region, the South West, and all but one of its eleven seats in Scotland, another traditional stronghold for the party.  Despite this, analysis in this post of the results of the election show that the large incumbency advantage the party has traditionally relied on has not gone away, but that it was not enough on its own to prevent a disaster.

At the election, the Liberal Democrat share of the vote fell in all 57 seats the party won in 2010.  The smallest decline was in East Dunbartsonshire (2.4%); the largest was in Brent Central (35.8%), with a mean decline of 15.7%, slightly worse than the average in Great Britain.  The table below shows the change in the Lib Dem vote from 2010 to 2015 in various categories of constituencies.


The key finding shown in the table is that there was a difference in change in the vote depending on whether there was incumbent or not.  The first block in the table shows the full 57 seats in Great Britain.  In 37 of these there was an incumbent at the 2010 election, and an incumbent re-standing in 2015.  In 10, the Lib Dem incumbent who was re-elected in 2010 retired before this election, or in one case did not stand as a Lib Dem.  In 9, the Lib Dems did not have an incumbent MP at the previous election, but had one re-standing for the first time in 2015, and in one case, Redcar, there was a first term incumbent who chose not to re-stand.

The results show that where an existing incumbent stood down, the party’s performance was far worse, a drop of 21.3% versus a drop of 15.7% in all previously held seats.   This 5.6% difference is an estimate of the retirement slump I have written about previously.   Meanwhile, the first term incumbents who had had five years to build up their personal vote in the constituency, which had not been reflected in the 2010 results, saw the best performance with a mean drop of 9.8%.   This 5.9% difference is the so-called sophomore surge.  The mean of the two, 5.75% is the co called “slurge” method estimate of incumbency advance and this is roughly the same as I estimated it for the 2010 election.

The second block of the table shows the results in the 43 English seats the Lib Dems won in 2010.   Performance in England was worse than in Scotland and Wales, with a mean 16.9% drop in the vote in seats won in 2010.   Performance in Scotland also appears to have been distorted by tactical voting in the party’s favour in three seats, and against the party in the Conservatives’ favour in Roxburgh.  Both sides of the incumbency factor had a larger effect in England with performance in seats with a retiring incumbent averaging a drop of almost 25%, whilst the average in seats with a first term incumbent was a drop of 10.7%.  The slurge estimate of incumbency in England is therefore estimated at 6.9%, up on what it was at the 2010 election.

Despite this continuing incumbency advantage, none of the first term incumbents re-standing survived the election.  As the previous post warned, all of them were in vulnerable places, so whilst David Ward in Bradford East saw his vote decline by a relative modest 4%, he was defending a majority of only 365 votes.   Another irony is that results were so poor nationally that the 10 incumbent retirements were likely only to have cost the party one seat, much smaller than it would have been had the decline in the vote been smaller.The Conservative majority of 8.1% in Bath was slightly more than the 6.9% estimated Lib Dem own vote incumbency advantage, but as my previous research has shown at least half this Lib Dem incumbency advantage comes from otherwise Conservative voters, and so it appears highly likely that Don Foster’s retirement tipped the balance.


Given that the sophomore surge failed to save a single Lib Dem MP and the large number of retirements only made the difference in one seat, one might well ask how this is of interest to anyone other than academics.  However, I would argue that incumbency advantage is now critical to the party’s survival.  On the assumptions outlined for Bath, had any one of the seven surviving MPs other than Tim Farron decided to retire ahead of the 2010 election, the loss of incumbency advantage would have almost certainly meant the seat was lost.  This means that with the exception of Westmoreland & Lonsdale, should any of sitting MPs decide to stand down in 2020, the party will be effectively fighting the seat as the challenger before any votes are cast.  Should Nick Clegg decide to take a job at the United Nations that has been touted by some, the numbers suggest the party would have an enormous challenge to hold onto Sheffield Hallam. Some might argue given the scale of the losses under his watch, he might well owe his party ten or even twenty years service as a back-bencher keeping the seat in the party’s hands whilst it rebuilds.

The silver lining for the Lib Dems is that the fact that the incumbency advantage appears to be holding up suggests that their traditional recovery route, that of winning by-elections and then using incumbency to hold onto the seat may be an option if they are able to compete with UKIP and the Greens and win by-elections.  After the 1970 election, at which the Liberals were reduced to six seats, the party made five by election gains in the subsequent Parliament, three of which they held on to at the February 1974 election, and one, Berwick, which survived until this election.

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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