|Part of the Politics series|
A by-election (occasionally also spelled bye-election, and known in the United States, Palau, Micronesia and sometimes in Liberia  and the Philippines as a special election) is an election held to fill a political office that has become vacant between regularly scheduled elections.
Usually, a by-election occurs when the incumbent has died or resigned. It may also occur when the incumbent becomes ineligible to continue in office, for example because of a recall, ennoblement, a serious criminal conviction, failure to maintain a minimum attendance, bankruptcy or mental incapacity. By-elections have also been called as a result of a constituency election being invalidated by voting irregularities; see Oldham East and Saddleworth election petition.
Historically, members of some parliaments were required to seek re-election upon being appointed to a ministerial post. The subsequent by-elections were termed ministerial by-elections. These by-elections were usually a formality as they were normally, but not always, uncontested by opposition parties. The requirement for Members of Parliament (MPs) to resign their seats and re-offer upon being appointed to Cabinet was done away with as an anachronism in most Westminster systems by the mid-20th century.
In single-member constituencies 
By-elections are held in most nations that elect their parliaments through single-member constituencies, whether with or without a runoff round. This includes most Commonwealth countries, such as the United Kingdom, Canada and Pakistan, as well as non-Commonwealth countries such as France. In the United States they are called special elections, and are held when a seat in Congress, a state legislature or at the local level becomes vacant.
In multi-member constituencies 
When one seat in a proportional representation constituency becomes vacant, the consequences vary. For example, a by-election may be held to fill just the vacancy or all the seats in the constituency be contested in the by-election.
Scotland and New Zealand still hold by-elections, despite having adopted the mixed-member proportional representation system, in which members are chosen by party lists. In both countries, by-elections where voters elect their preferred candidate are only used to fill a vacancy in a constituency seat. For example, the death of Donald Dewar resulted in a by-election for the constituency of Glasgow Anniesland. If a vacancy arises from the death or resignation of a party list member, the next unelected candidate on the party list is offered the seat. If that candidate has died or declines the seat, it is offered to subsequent candidates on the list until one accepts the seat. For example, on the resignation of Darren Hughes in March 2011, Louisa Wall was elected after all the five candidates above her on the New Zealand Labour Party's list declined the seat. The Republic of Ireland holds by-elections despite electing members in multi-member constituencies by the single transferable vote.
Alternatives to holding a by-election include:
- choosing from those losing candidates at the previous election who choose to contest the recount to fill the vacancy, as in Tasmania or the Australian Capital Territory,
- keeping the seat vacant until the next general election. This usually occurs if a vacancy arises shortly before a planned general election (within six months in New Zealand).
- nominating another candidate with the same affiliation as the former member – typically, in list systems, the next candidate on the party list.
For the Australian Senate (where each State forms a multi-seat constituency voting by single transferable vote), the State Parliament appoints a replacement; in 1977 a referendum amended the Constitution to require that the person appointed must belong to the same political party (if any) as the Senator originally elected to that seat. Those Australian states with an Upper House elected by PR-STV (NSW, Victoria and South Australia) copy the federal Senate model, except for Western Australia, which holds a recount of ballots, with sitting members retaining their seats.
By-elections can be crucial when the ruling party has only a small majority. In parliamentary systems, party discipline is often so strong that the governing party can only lose a vote of no confidence after losing enough by-elections for it to become a minority government. Examples are the Labour government of James Callaghan 1976–1979 and the Conservative government of John Major 1992–1997. In the United States, Scott Brown's election in 2010 ended the filibuster-proof majority formerly enjoyed by Democrats.
By-elections can also be important if a minority party needs to gain one or more seats in order to gain official party status or the balance of power in a minority or coalition situation. For example, Andrea Horwath's win in an Ontario provincial by-election in 2004 allowed the Ontario New Democratic Party to regain official party status with important results in terms of parliamentary privileges and funding.
Indirect impact 
By-election upsets can have a psychological impact by creating a sense of momentum for one party or a sense of impending defeat for a government. Deborah Grey's 1989 by-election victory in Beaver River was seen as evidence that the newly formed Reform Party of Canada would be a serious political contender and that it posed a serious political threat for the ruling Progressive Conservatives. It also gave important momentum to the new party. Similarly, the upset 1960 by-election victory of Walter Pitman in Peterborough as a "New Party" candidate was a significant boost for the movement to replace the Co-operative Commonwealth Federation with an unnamed "New Party" which would be integrated with the labour movement. Pitman's candidacy in a riding in which the CCF was traditionally weak was seen as a test of this concept and his upset victory convinced the CCF and the labour movement to launch the New Democratic Party (NDP).
By-elections may occur singly, or in small bunches, especially if the authority responsible for calling them has discretion over the timing and can procrastinate. They are sometimes bunched to save money, as holding multiple by-elections is likely to cost more than holding a by-election to fill the vacancies all at once. In Canada, in 1978, 15 by-elections were held on a single date, restoring the House of Commons to 264 members. The media called it a "mini-election", a test of the Liberal government's popularity with a general election due in less than a year. The 15 districts stretched from Newfoundland to British Columbia, and produced some unexpected results, for example, an NDP candidate winning in Newfoundland for the first time.
In Canada, the most recent example of a cabinet minister appointed from outside of parliament having to resign after losing a by-election was in 1975 when Minister of Communications Pierre Juneau was appointed to Pierre Trudeau's Liberal cabinet directly from the private sector and tried to enter parliament through a by-election in Hochelaga. Juneau was upset by the Progressive Conservative candidate and resigned from cabinet ten days after his by-election defeat.
General Andrew McNaughton was appointed to Cabinet as Minister of Defence on November 1, 1944 without having a seat in parliament after his predecessor resigned during the Conscription Crisis of 1944. A by-election was arranged in Grey North which the opposition Progressive Conservative party contested. The major campaign issue became the government's policy of "limited conscription" during World War II which McNaughton supported and which the Conservatives counterposed with a call for "full conscription". McNaughton was upset in the February 5, 1945 by-election. As a result, with confidence in his government undermined, Prime Minister William Lyon Mackenzie King called the 1945 federal election several weeks later when he had originally intended to wait until after the end of the war. McNaughton sought a seat in the federal election and resigned after he was again defeated.
In 1942, new Conservative Party leader Arthur Meighen sought to enter the Canadian House of Commons through a by-election in York South. His surprise defeat at the hand of Joseph Noseworthy of the Co-operative Commonwealth Federation ended his political career, and may also have been a factor in the Conservative Party's decision to move to the left and rebrand itself the Progressive Conservative Party under Meighen's replacement. Noseworthy's victory was also a significant breakthrough for the CCF giving it credibility as a national party where it has previously been seen as a Western Canadian regional protest party.
In the Canadian province of Ontario, John Tory, leader of the Progressive Conservative Party of Ontario ran in a 2009 by-election in Haliburton—Kawartha Lakes—Brock, after he convinced one of his caucus members to step down, in hopes of re-entering the Ontario legislature. His by-election defeat resulted in his resignation as party leader.
A Massachusetts special Senate election held in January 2010 produced a significant upset when Republican Scott Brown won the United States Senate seat formerly held for 48 years by Democratic Party stalwart Ted Kennedy. Republicans termed the result the "Massachusetts Miracle" 
1986 Northern Ireland by-elections 
In Northern Ireland there were 15 by-elections held on 23 January 1986 to fill vacancies in the Parliament of the United Kingdom caused by the resignation in December 1985 of all the sitting Unionist Members of Parliament (MPs). The MPs, from the Ulster Unionist Party, Democratic Unionist Party and Ulster Popular Unionist Party, resigned to highlight their opposition to the Anglo-Irish Agreement. Each of their parties agreed not to contest seats previously held by the others, and each outgoing MP stood for re-election. All but one of the Unionists were re-elected, many with extremely large majorities, against pro-Agreement or in some cases Irish Republican opponents. In some seats, a fictitious candidate named after Peter Barry, the Irish foreign minister, appeared on the ballot paper. The largest of all majorities went to Ian Paisley in North Antrim. He won 97.4% of the vote, the highest percentage polled by any candidate in a UK by-election since the 1940 Middleton and Prestwich by-election.
The sole exception to this pattern was the Newry and Armagh by-election, where Seamus Mallon of the Irish nationalist and pro Anglo-Irish Agreement Social Democratic and Labour Party (SDLP) took the seat. The results of the 15 by-elections were cited by Unionists as a rejection of the Agreement by the Northern Irish electorate, but the action did not succeed in persuading the government of Margaret Thatcher to repeal the accord.
2010 Hong Kong by-elections 
Territory-wide by-elections were held on 16 May 2010 in Hong Kong. The by-elections were triggered by the resignation of five members of the Legislative Council, one each from the territory's five constituencies, three of them from the League of Social Democrats, and the other two from the Civic Party. It was meant to be a de facto referendum on the territory's electoral reform.
Thirty seats in the 60-seat legislature are elected from "geographical constituencies". As in 2010 there were five multi-member constituencies. Elections are held using the largest remainder method of the proportional representation system. A by-election is necessary whenever a seat is vacant.
See also 
- UK Parliamentary by-elections
- List of democracy and elections-related topics
- Lists of by-elections and special elections by jurisdiction
- Australia: List of Australian federal by-elections
- Canada: List of federal by-elections in Canada
- Ireland: List of Irish by-elections
- New Zealand: New Zealand by-elections
- Philippines: List of special elections to the Philippine Congress
- Solomon Islands: List of Solomon Islands by-elections
- United Kingdom: List of United Kingdom by-elections
- United States: List of special elections to the United States House of Representatives
- United States: List of special elections to the United States Senate
- United States: List of United States special gubernatorial elections
- Re-election was required under the Succession to the Crown Act 1707. The Re-Election of Ministers Act 1919 severely restricted the necessity to seek re-election on appointment to government office. The Re-Election of Ministers Act (1919) Amendment Act 1926 ended the practice. http://www.election.demon.co.uk/causes.html (19/10/2012)
- "Tasmania's Hare-Clark Electoral System".
- "Casual Vacancies in the Legislative Assembly".
- "Power Line - More historical perspective on the Massachusetts miracle". Powerlineblog.com. Retrieved 2010-01-23.
- 10:12 EST (2010-01-11). "Hugh Hewitt: A Massachusetts miracle?". The Washington Examiner. Retrieved 2010-01-23.
- Malkin, Michelle (2010-01-19). "Massachusetts Miracle: Election night liveblog; Update: BROWN WINS, COAKLEY CONCEDES; Brown victory speech cheers: "41!41!41!"". Michelle Malkin. Retrieved 2010-01-23.