Vote splitting is an electoral effect in which the distribution of votes among multiple similar candidates reduces the chance of winning for any of the similar candidates, and increases the chance of winning for a dissimilar candidate. This is commonly known as the spoiler effect, which can discourage minor party candidacies.

Vote splitting most easily occurs in plurality voting (also called first-past-the-post) in which each voter indicates a single choice and the candidate with the most votes wins, even if the winner does not have majority support. For example, if candidate A1 receives 30% of the votes, similar candidate A2 receives another 30% of the votes, and dissimilar candidate B receives the remaining 40% of the votes, plurality voting declares candidate B as the winner, even though 60% of the voters prefer either candidate A1 or A2.

Occurrence One of the main functions of political parties is to mitigate the effect of spoiler-prone voting methods by winnowing on a local level the contenders before the election. Each party nominates at most one candidate per office since each party expects to lose if they nominate more than one. This means empirical observations of the frequency of spoiled elections may not be a good measure, because they exclude relevant information from candidates who chose not to run.

Vote splitting occurs when candidates or ballot questions have similar ideologies. A spoiler candidate can draw votes from a major candidate with similar politics, thereby causing a strong opponent of both or several to win. The minor candidates causing this effect are referred to as spoilers.

The problem also exists in two-round system and instant-runoff voting, since candidates are eliminated based on tallies of first-choice preferences, which get split between similar candidates, though it is reduced, because weaker spoilers are eliminated. All ranked-choice systems suffer from variations of the spoiler effect, according to Arrow's impossibility theorem. However, a candidate that can win head-to-head against all rivals (called a Condorcet winner) can still lose from third place in a 3-way vote split, a phenomenon known as a center squeeze. This occurred in the 2009 Burlington Vermont mayoral election and the 2022 Alaska's at-large congressional district special election.

List of systems designed to reduce vote splitting Approval voting Citizens' assembly Electoral fusion Proportional representation

Election examples by country

Australia In Australia, the 1918 Swan by-election saw the conservative vote split between the Country Party and Nationalist Party, which allowed the Australian Labor Party to win the seat. That led the Nationalist government to implement preferential voting in federal elections to allow Country and Nationalist voters to transfer preferences to the other party and to avoid vote splitting. Today, the Liberal Party and National Party rarely run candidates in the same seats, which are known as three-cornered contests. When three-cornered contests do occur the Labor Party would usually direct preferences to the Liberals ahead of the Nationals as they considered the Liberal Party to be less conservative than the Nationals. The 1996 Southern Highlands state by-election in New South Wales is an example of this when the Nationals candidate Katrina Hodgkinson won the primary vote but was defeated after preferences to Liberal candidate Peta Seaton when Seaton received Labor Party preferences.

Bolivia Movimiento al Socialismo (MAS) criticised the registration of Pachakuti Indigenous Movement (MIP) claiming that it was done to split the Indigenist vote in the 2002 general election. The two parties discussed an electoral alliance before both parties were allowed to run. MAS lost by less than 2% to the traditional Revolutionary Nationalist Movement (MNR) while MIP unexpectedly won more than 6%. Combined the two parties had 27.03% compared to the 22.46% of MNR.

Bulgaria

In 2001, the former tsar of Bulgaria Simeon II founded the NDSV. The NDSV won exactly 50% of the seats (120 out of 240 seats) thus barely missing an outright majority. Similarly named parties "Simeon II" Coalition, "National Union for Tsar Simeon II", "National Union Tsar Kiro" Coalition and "National Movement for New Era" (NDNE) got 3.44%, 1.70%, 0.60% and 0.05% respectively.

In Bulgaria, the so-called "blue parties" or "urban right" which include SDS, DSB, Yes, Bulgaria!, DBG, ENP, Coalition For you Bulgaria and Blue Unity frequently get just above or below the electoral threshold depending on formation of electoral alliances: In the EP election 2007, DSB (4.74%) and SDS (4.35%) were campaigning separately and both fell below the natural electoral of around 5 percent. In 2009 Bulgarian parliamentary election, DSB and SDS ran together as Blue Coalition gaining 6.76 percent. In 2013 Bulgarian parliamentary election, campaigning separately DGB received 3.25 percent, DSB 2.93 percent, SDS 1.37 percent and ENP 0.17 percent, thus all of them failed to cross the threshold this even led to a tie between the former opposition and the parties right of the centre. In the EP election 2014, SDS, DSB and DBG ran as Reformist Bloc gaining 6.45 percent and crossing the electoral threshold, while Blue Unity campaigned separately and did not cross the electoral threshold. In 2017 Bulgarian parliamentary election, SDS and DBG ran as Reformist Bloc gaining 3.14 percent, "Yes, Bulgaria!" received 2.96 percent, DSB 2.54 percent, thus all of them failed to cross the electoral threshold. In the EP election 2019, "Yes, Bulgaria!" and DBG ran together as Democratic Bulgaria and crossed the electoral threshold with 6.15 percent. In November 2021, electoral alliance Democratic Bulgaria crossed electoral threshold with 6.37 percent.

+ Parties2005EP 2007EP 200920092013EP 201420142017EP 2019April 2021July 2021November 20212022VotesSeatsVotesSeatsVotesSeatsVotesSeatsVotesSeatsVotesSeatsVotesSeatsVotesSeatsVotesSeatsVotesSeatsVotesSeatsVotesSeatsVotesSeatsDBGcolspan=16 115,190 3.25 0 0.00144,532 6.45 1 5.88291,806 8.89 23 9.58107,407 3.14 0 0.00colspan=20 ODS/SDS280,323 7.68 20 8.3391,871 4.74 0 0.00 204,817 7.95 1 5.88285,662 6.76 15 7.18proportional seats48,681 1.37 0 0.00colspan=20 DSB234,788 6.44 17 7.0884,350 4.35 0 0.00103,638 2.93 0 0.0086,984 2.54 0 0.00118,484 6.06 1 5.88302,280 9.45 27 11.25345,331 12.64 34 14.17166,968 6.37 16 6.66186,528 7.45 20 8.33DBcolspan=28 101,177 2.96 0 0.00ENPcolspan=16 6,143 0.17 0 0.00colspan=4 7,2340.2200.00colspan=4 1,855 0.09 0 0.00colspan=16 Blue Unitycolspan=20 10,786 0.48 0 0.00colspan=24 colspan=36 4,788 0.15 0 0.00colspan=8 5,097 0.20 0 0.00Total515,111 14.12 37 15.42176,221 9.09 0 0.00204,817 7.95 1 5.88285,662 6.76 15 7.18382,699 10.08 0 0.00155,318 6.93 1 5.88299,040 9.10 23 9.58295,568 8.62 0 0.00120,339 6.15 1 5.88307,068 9.60 27 11.25345,331 12.64 34 14.17166,968 6.37 16 6.66191,625 7.65 20 8.33Source:CIKCIKCIKCIKCIKCIKCIKCIKCIKCIKCIKCIKCIK

Canada When the cities of Fort William and Port Arthur merged and (in 1969) voted on a name for the new town, the vote was split between the popular choices of "Lakehead" and "The Lakehead", allowing the third option to win, creating the town of Thunder Bay, Ontario.About Thunder Bay, pp. 2. Retrieved 2 September 2007.

From 1993 to 2004, the conservative vote in Canada was split between the Progressive Conservatives and the Reform (later the Alliance) Party. That allowed the Liberal Party to win almost all seats in Ontario and to win three successive majority governments.

The 2015 provincial election in Alberta saw the left-wing New Democratic Party win 62% of the seats with 40.6% of the province's popular vote after a division within the right-wing Progressive Conservative Party, which left it with only 27.8% of the vote, and its breakaway movement, the Wildrose Party, with 24.2% of the vote. In 2008, the last election in which the Progressive Conservative Party had been unified, it won 52.72% of the popular vote. The Progressive Conservatives had won every provincial election since the 1971 election, making them the longest-serving provincial government in Canadian history—being in office for 44 years. This was only the fourth change of government in Alberta since Alberta became a province in 1905, and one of the worst defeats a provincial government has suffered in Canada. It also marked the first time in almost 80 years that a left-of-centre political party had formed government in Alberta since the defeat of the United Farmers of Alberta in 1935 and the Depression-era radical monetary reform policies of William Aberhart's Social Credit government. During the 2021 Canadian federal election, it is speculated that the People's Party of Canada might have coast the CPC up to 24 seats.

In Canada, vote splits between the two major left-of-centre parties (Liberals and NDP) assisted the Conservative Party in winning the 2006, 2008, and 2011 federal elections, despite most of the popular vote going to left-wing parties in each race. During the 2022 Ontario General Election, Progressive Conservative Doug Ford won a second term as Premier of the Province of Ontario. The Progressive Conservatives won several ridings due to vote splitting. ONDP and Liberal Party voters combined for 47.8% of votes, whereas Ford emerged victorious with only 40.82% of total votes.

Chile In the 2025 Chilean general election, multiple left-wing parties did not join the Unidad por Chile but competed with their own lists, namely the Greens, Regionalists and Humanists, the Popular Ecologist, Animalist, and Humanist Left, the Popular Green Alliance Party, People's Party and Green Ecologist Party. Combined these lists would have won 10% of the vote but only the first list managed to win 3 seats.

Similarly, both Amplitude and Amarillos por Chile endorsed the candidate of Chile Vamos but did not join the coalition and ended up without seats.

+ +Valparaíso Region in the 2017 electionCoalition/1/2/3Chile Vamos244,623 (1) (3)with Amplitude (1) (3) (5)The Force of Majority182,799 (2) (5)Frente Amplio96,394 (4)Source+District 5 in the 2025 electionCoalition/1/2/3/4Unidad por Chile161,027 (1) (2) (6) (7)Chile Vamos76,051 (4)with Amarillos (2) (7)Change for Chile76,992 (3)Party of the People64,845 (5)Source+District 20 in the 2025 electionCoalition/1/2/3/4Change for Chile154,584 (1) (5) (7)Unidad por Chile128,105 (2) (6)VRHGreen Ecologist PartyPopular Ecologist Leftcombined (3) (8)Chile Vamos98,546 (3) (8)Party of the People78,461 (4)Sourceitalic: seats effected by vote-splitting

Czech Republic After Independents disintegrated, their MEPs Vladimír Železný and Jana Bobošíková ran for Libertas and Sovereignty - Jana Bobošíková Bloc, alongside Party of Free Citizens three eurosceptic and Klausist lists ran in the election splitting the vote.

In 2021, Přísaha (4.68%), ČSSD (4.65%) and KSČM (3.60%) all failed to cross the 5 percent threshold, thus allowing a coalition of Spolu and PaS. This was also the first time that neither ČSSD nor KSČM had representation in parliament since 1992.

Egypt

In the 2012 Egyptian presidential election, held using the two-round system, vote-splitting among three leading moderate, non-Islamist candidates caused them all to be eliminated in the first round. This allowed the two more polarizing candidates, Mohamed Morsi and Ahmed Shafik, to advance to the runoff, despite pre-election polls suggesting the eliminated moderates would have defeated either finalist in a head-to-head contest. This led to the June 2013 Egyptian protests and 2013 Egyptian coup d'état. France