From Citizendium, the Citizens' Compendium
Deselection in British politics refers to the refusal of a local party association to continue supporting a candidate who had previously been elected under their banner. The practice is relatively rare in Parliamentary elections. It can occur for a variety of reasonS. The three most common explanations are: (a) where boundary changes have merged the power bases of two Members, (b) where the Member is felt to be too old to continue to represent the seat effectively, and (c) where political differences have opened up between the Member and their association. In a few cases, Members who have been deselected have subsequently been re-elected as an Independent candidate or for another party.
Prior to the introduction of the secret ballot between 1872 and 1885, there was no need for local associations formally to select a candidate. The fact that the poll was in public meant that the state of the poll was known and so it was not possible for rival candidates to accidentally 'split the vote' and let the seat be taken by on a minority.
When the secret ballot came in, local political associations began to hold private meetings before the poll to choose between those who wished to represent the party. The winner of this ballot would then receive official endorsement in the form of supportive speeches from their party's national figures, letters of endorsement, and the assistance of a campaign team. Where the loser of such a ballot decided to contest the election despite not having official endorsement, they often received a derisory vote: for example, in Paddington South in 1885, the official Liberal candidate Hilary Skinner won 1,025 votes, while his defeated Alderman Lawrence had only 290.
The procedure when a sitting Member came to be renominated was normally a formality. A legal technicality not repealed until 2005 made a difference between a 'selected prospective candidate' (one who had been picked as the provisional choice for a forthcoming election) and an 'adopted candidate' (one who had definitely been chosen to fight the election): campaigning by the former did not count towards a limit on election spending. Sitting Members would not have to go through a selection procedure but would merely be adopted on the eve of an election, by which time it was divisive to open up a debate on their merits, and it would be too late to pick an alternative. Readoption was usually unanimous, as a way of expressing confidence in a sitting Member.
Operation in 1886
In the 1886 general election the value of this system was appreciated as a large number of Liberal MPs had seceded from the leadership of Gladstone and offered themselves as Liberal Unionists. The Conservative Party nationally refused to endorse any Conservative running in opposition to a Liberal Unionist, which ensured a united Unionist vote.  Meanwhile many of the Liberal Party's associations attempted to find candidates of their own to run against the Liberal Unionists, on the grounds of their secession from the Party. 
However, not all sitting Liberal MPs who lost the endorsement of their party did so purely because of their stance on the Irish question. John Westlake (Romford), had on March 23, 1886 voted to oppose a motion put forward by another Liberal which favoured the burden of local taxation falling on owners rather than occupiers of land. This vote was strongly resented by the Romford Liberal Association, and on June 10 a meeting of local Liberals instructed their executive to find another candidate. 
Subsequent 19th century developments
No Member suffered outright repudiation at the 1892 general election. There were rumours that the sitting Conservative MP for Evesham, Sir Richard Temple, was encouraged to step down because his constituency association felt that his indifferent health and duties at the London School Board made the seat vulnerable; he was adopted for a Kingston-upon-Thames instead.
The Labour left in the 1970s
In 1973 the Labour Party adopted a left-wing policy statement called "Labour's Programme, 1973" which was immediately dismissed by party leader Harold Wilson. Wilson's action led to the establishment of the Campaign for Labour Party Democracy (CLPD), which aimed to increase the power of the membership of the party.  At the 1973 Labour Party conference, a CLPD meeting heard demands from Lincoln Constituency Labour Party members for a new policy of "mandatory reselection".  Under this policy, each sitting Labour MP who wished to continue would have to be approved by the Constituency Labour Party, with no procedural advantages over outside candidates.
From 1974, mandatory reselection became CLPD policy and the campaign distributed 'model resolutions' for supportive Constituencies to send to the Labour Party conference in order to get the Labour Party constitution changed. CLPD's influence increased, and the 12 CLPs sending mandatory reselection resolutions for the 1975 conference increased to 45 in 1976 and 79 in 1977, when the issue was selected for debate in a private session of the conference. The motion was moved by Ray Apps of Brighton Kemptown CLP, a supporter of the Militant Tendency. The Labour Party's National Executive Committee persuaded conference to remit the motion for reconsideration (by 4,858,000 to 1,560,000), on an understanding that the NEC itself would propose mandatory reselection at the 1978 conference. 
Shortly after the 1977 conference the campaign for mandatory reselection had a significant boost. Supporters had long argued that many sitting Labour MPs had become out of touch with their local parties, and the voters. Reginald Prentice, who was Minister for Overseas Development, was asked to retire by his Newham North East CLP on July 23, 1975 on a vote of 29 to 19. Prentice fought the deselection by appealing to the NEC, and on his behalf two activists (Paul McCormick and Julian Lewis) moved into Newham North East to fight his corner. However, on October 8, 1977 (the weekend after the Labour Party conference) Prentice announced that he had joined the Conservative Party.
At Labour's 1978 conference, the NEC went back on its pledge to bring forward its own proposals for mandatory reselection. Instead, it proposed a simpler version of the existing system. An amendment supporting mandatory reselection was moved and debated in open session, but lost by 2,672,000 to 3,066,000 after a debate in which Michael Cocks (the Chief Whip) argued that the strains put on Labour MPs by their constituency parties were becoming unsustainable. The announcement of the loss was followed by a point of order from Hugh Scanlon of the Amalgamated Union of Engineering Workers who complained that he had not understood which motion was being voted on, and had mistakenly cast his union's block vote against mandatory reselection instead of for.  Had Scanlon voted the way he intended to, the amendment would have been carried, and it was suspected on the left that his 'accident' was in fact deliberate. 
Other than Reginald Prentice, the only Labour MP actually deselected before the 1979 general election was Frank Tomney (Hammersmith North). In addition, Sir Arthur Irvine (Liverpool Edge Hill), was deselected but died in December 1978; the seat was won by a Liberal in a March 1979 byelection.
Labour in opposition
The pressure from constituency parties for a debate at the 1979 Labour Party conference was so great that the Labour Party allowed one despite a rule prohibiting a defeated constitutional amendment from being brought back within three years. Mandatory reselection was passed in 1979 by 4,521,000 to 2,356,000. 
Many expected that the introduction of mandatory reselection would lead to a very large number of sitting Labour MPs being deselected. Joe Ashton, speaking at Labour Party conference, declared he was appealing to save the jobs of Labour MPs by arguing against mandatory reselection.
- ↑ "T.G.P.H." in a letter to The Times published on May 10, 1886, wrote of "The unseemly and unprofitable sight .. of one Unionist party running a candidate against the other's previously sitting Member". However there were individual exceptions: Conservatives in Torquay were told by Lord Salisbury that they should oppose the sitting Liberal MP, McIver, who remained close to Gladstone despite his vote against Home Rule. The Conservatives won the seat.
- ↑ There were again exceptions: among them were W. Cuthbert Quilter in Sudbury, who was given a vote of confidence by 80 to 2; A.H. Brown in the Wellington division of Shropshire survived by 144 to 56.
- ↑ "An Elector" wrote to The Times on June 18 (published on June 21) to note that "This is an actual case of a Liberal member who had forfeited the confidence and support of his party before ever the Home Rule Bill was launched".
- ↑ David Kogan and Maurice Kogan, "The Battle for the Labour Party", Fontana Paperbacks 1982, p. 23.
- ↑ Vladimir Derer, quoted in "The Battle for the Labour Party", p. 26.
- ↑ "Report of the Seventysixth Annual Conference of the Labour Party", 1977, p. 324.
- ↑ "Report of the Seventyseventh Annual Conference of the Labour Party", 1978, p. 281.
- ↑ Jon Lansman, one of CLPD's organisers, is quoted in "The Battle for the Labour Party" (p. 34) as saying "This was seen as a scandalous abstention. It created an uproar and we organised a massive campaign on the lines of 'shock, horror, we wuz robbed'."
- ↑ Patrick Seyd, "The Rise and Fall of the Labour Left" (Macmillan, 1987), p. 109.