Mixed-member proportional representation is a hybrid two-tier voting system. MMP was originally used to elect representatives to the German Bundestag, and has been adopted by Bolivia, Lesotho and New Zealand. It was used in Romania during its 2008 and 2012 legislative elections. MMP is a hybrid method that uses party list proportional representation (PR) as its proportional component, and most commonly (but not necessarily) first-past-the-post voting (FPTP) as its district component. It is considered a mixed system (also known as a hybrid system or a semi-proportional representation system), which is a distinct voting system. "An electoral system is 'mixed' if more than one formula is employed to distribute legislative seats." Biproportional apportionment, first used in Zürich in 2006, is a hybrid method for adjusting an election's result to achieve overall proportionality.
In Germany, where it is used on the federal level and on most state levels, MMP is known as personalized proportional representation (German: personalisiertes Verhältniswahlrecht). In the United Kingdom such systems used in Scotland, Wales, and the London Assembly are referred to as Additional Member System. In the Canadian province of Quebec, where an MMP model was studied in 2007, it is called the compensatory mixed-member voting system (système mixte avec compensation or SMAC).
In most models the voter casts two votes: one for a constituency representative and one for a party. In the original variant used in Germany, both votes were combined into one, so that voting for a representative automatically meant also voting for the representative's party. Most of Germany changed to the two-vote variant to make local members of parliament (MPs) more personally accountable. Voters can vote for the local person they prefer for local MP without regard for party affiliation, since the partisan make-up of the legislature is determined only by the party vote. In the 2005 New Zealand election, 20% of local MPs were elected from electorates (constituencies) which gave a different party a plurality of votes.
Most systems used closed party lists to elect the non-constituency MPs (also called list MPs). Depending on the jurisdiction, candidates may stand for both a constituency and on a party list (referred to in New Zealand as dual candidacy), or may be restricted to contend either for a constituency or for a party list, but not both. If a candidate is on the party list, but wins a constituency seat, they do not receive two seats; they are instead crossed off the party list and replaced with the next candidate down.
In Bavaria the second vote is not simply for the party but for one of the candidates on the party's regional list: Bavaria uses seven regions for this purpose. A regional open-list method was also recommended for the United Kingdom by the Jenkins Commission (where it is known as AMS) and for Canada by the Law Commission of Canada. The open-list method of MMP was also chosen in November 2016 by voters in the Prince Edward Island electoral reform referendum, 2016.
In Baden-Württemberg there are no lists; they use the "best near-winner" method in a four-region model, where the regional members are the local candidates of the under-represented party in that region who received the most votes in their local constituency without being elected in it.
At the regional or national level (i.e. above the constituency level) several different calculation methods have been used, but the basic characteristic of the MMP is that the total number of seats in the assembly, including the single-member seats and not only the party-list ones, are allocated to parties proportionally to the number of votes the party received in the party portion of the ballot. This can be done by the largest remainder method or a highest averages method: either the D'Hondt method or the Sainte-Laguë method. Subtracted from each party's allocation is the number of constituency seats that party won, so that the additional seats are compensatory (top-up). If a party wins more FPTP seats than the proportional quota received by the party-list vote, these surplus seats become overhang seats to work towards restoring a full proportionality. In most German states, but not federally until the federal election of 2013, "balance seats" are added to compensate for the overhang seats and achieve complete proportionality. In the last election in Scotland, the highest averages method resulted in a majority government for the Scottish National Party with only 44% of the party vote. However, Scotland uses the term Additional Member System which, like MMP can either be proportional or semi-proportional.
When a party wins more constituency seats than it would be entitled to from its proportion of (party list) votes, overhang seats can occur.
In Germany's Bundestag and the New Zealand House of Representatives, all these constituency members keep their seats. For example, in New Zealand's 2008 General Election the Māori Party won 2.4% of the Party Vote, which would entitle them to 3 seats in the House, but won 5 constituency seats, leaving an overhang of 2 seats, which resulted in a 122-member house. If the party vote for the Māori Party had been more in proportion with the constituency seats won, there would have been a normal 120-member house.
In most German states, and in the federal Bundestag since 2013, the other parties receive extra seats ("balance seats") to create full proportionality. For example, the provincial parliament (Landtag) of North Rhine Westphalia has, instead of the usual 50% compensatory seats, only 29% unless more are needed to balance overhangs. If a party wins more local seats than its proportion of the total vote justifies, the size of the Landtag increases so that the total outcome is fully proportional to the votes, with other parties receiving additional list seats to achieve proportionality.
As in numerous proportional systems, in order to be eligible for list seats in many MMP models, a party must earn at least a certain percentage of the total party vote, or no candidates will be elected from the party list. Candidates having won a constituency will still have won their seat. In New Zealand the threshold is 5%, in Bolivia 3%, in Germany 5% for elections for federal parliament and most state parliaments. A party can also be eligible for list seats if it wins at least three constituency seats in Germany, or at least one in New Zealand. Having a member with a 'safe' constituency seat is therefore a tremendous asset to a minor party in New Zealand.
MMP is currently in use in:
In March 2004 the Law Commission of Canada proposed a system of MMP, with only 33% of MPs elected from regional open lists, for the Canadian House of Commons but Parliament’s consideration of the Report in 2004-5 was stopped after the 2006 election.
The New Democratic Party of Canada has been a longtime supporter of MMP.
In 2007 the Citizens' Assembly on Electoral Reform in Ontario, Canada, also recommended the use of MMP in future elections to the Legislative Assembly of Ontario, with a ballot similar to New Zealand's, and with the closed province-wide lists used in New Zealand but with only 30% compensatory members. A binding referendum on the proposal, held in conjunction with the provincial election on 10 October 2007, saw it defeated.
In June 2016, the Canadian House of Commons Special Committee on Electoral Reform was formed to examine potential changes to the voting system with MMP being one of the options examined. The committee presented its report to Parliament on 1 December of the same year. In early 2017, the Government announced that it would accept only some of the committee's recommendations, and would not pursue the issue of electoral reform any further.
In a non-binding plebiscite between 27 October and 7 November, Prince Edward Islanders voted for MMP over FPTP in the final round of counting, 52%–43%; however, the provincial government, despite having set no voter turnout threshold, subsequently deemed that the 36 percent turnout was insufficient to change the electoral system.
In 2015, Thailand's Constitutional Drafting Committee proposed use of MMP for future national elections.
In systems with a threshold, people who prefer a larger party commonly tactically vote for a minor party that is predicted to poll close to or slightly below the threshold. Some voters may be afraid the minor party will poll below the threshold, and that that would weaken the larger political camp to which the minor party belongs. For example, the German moderate-right Free Democratic Party (FDP) has often received votes from voters who preferred the larger Christian Democratic Union (CDU) party, because they feared that if the FDP received less than 5% of the votes, the CDU would have no parliamentary allies and would be unable to form a government on its own. This tactical voting also ensures that fewer votes are wasted, but at the cost of giving the FDP more seats than CDU voters would ideally have preferred. This tactic is the same in any method of proportional representation.
Similarly, in New Zealand, some voters who preferred a large party have voted for the minor party's local candidate to ensure it qualifies for list seats on the back of winning a single electorate. This notably occurred in the right-wing inner Auckland electorate of Epsom in 2008 and 2011, where the National Party voters gave their local vote to the ACT Party. In this case the tactic maintained some proportionality by bypassing the 5% threshold, but is largely disfavoured by the public due to it awarding smaller parties extra list seats while parties with a higher party vote percentage that don't win an electorate receive no seats; this occurred in 2008 when ACT was awarded 5 seats on the back of one electorate seat and 3.7% of the party vote, while New Zealand First with no electorate seats and 4.1% of the party vote were awarded none. In 2011, some Epsom voters voting for the left-wing Labour and Green parties tried to block the tactic by giving their local vote to the National candidate; while it was unsuccessful, it did reduce ACT's majority over National from 12,900 to 2,300. In August 2012, the initial report on a review of the MMP system by the Electoral Commission recommended abolishing the one electorate seat threshold, meaning a party winning an electorate seat but not crossing the 5% threshold (which the same report recommends lowering to 4%) is only awarded that electorate seat.
In other cases a party may be so certain of winning a large number of constituency seats that it expects no extra seats in the proportional top-up (list seats). Some voters may therefore seek to achieve double representation by voting tactically for another party in the regional vote, as a vote for their preferred party in the regional vote would be wasted. This tactic is much less effective in MMP models with a relatively large share of list seats (50% in most German states, and 42.5% in the New Zealand House of Representatives) and/or ones which add "balance seats", leading to less opportunities for overhangs and maintaining full proportionality even when a party wins too many constituency seats.
This sort of strategy for a coalition of parties to capture a larger share of list seats may be adopted formally as a strategy. By way of example, in Albania's 2005 parliamentary election, the two main parties did not expect to win any list seats, so they encouraged voters to use their list votes for allied minor parties. This tactic was used to such an extent that it totally distorted the working of the model, to the point that the parties that won list seats were almost always different from the parties that won constituency seats. Indeed, only one constituency member was elected from parties that won list seats. Rather than increasing the number of list seats or "overhang" seats, Albania subsequently decided to change to a pure-list system.
In an abusive gambit similar to that used in Albania, major parties feeling that they are unlikely to win a large number of list seats because of their advantage at the constituency level might choose to split their party in two, with one subdivision of the party contesting the constituency seats, while the other contests the list seats —assuming this is allowed by electoral law. The two linked parties could then co-ordinate their campaign and work together within the legislature, while remaining legally separate entities. The result of this approach, if it is used by all parties, would be to transform MMP into a de facto Mixed Member Majoritarian MMM or Parallel voting mechanism. The election itself was condemned by the Organization for Security and Co-operation in Europe which said it failed to comply with international standards because of “serious irregularities,” intimidation, vote-buying and “violence committed by extremists on both sides.)
An example of how this could happen manifested itself in the 2007 Lesotho general election. In this case the two leading parties, the Lesotho Congress for Democracy (LCD) and the All Basotho Convention (ABC) used decoy lists, respectively named the National Independent Party and the Lesotho Workers' Party to avoid the compensatory mechanisms of MMP. As a result, the LCD and its decoy were able to take 69.1% of the seats with only 51.8% of the vote. ABC leader Tom Thabane called the vote "free, but not fair." In the 2012 election, the voting system was adjusted to link the local and list seats to limit the decoy lists' effectiveness, resulting in an almost perfectly proportionate election result for the competing parties.
Another interesting case is that of Venezuela, which also ended up changing its system, in this case by formally adopting a parallel voting system and legitimizing party efforts to game the MMP approach. Venezuela introduced an MMP electoral system in 1993, but the tactic of creating a decoy party was introduced only in 2000, by the opposition governor of Yaracuy. The tactic was later adopted by pro-Chavez parties at the national level in 2005. After the decoy list tactic withstood a constitutional challenge, Venezuela eventually formally reverted to a parallel voting system, which yields a lesser degree of proportionality compared to MMP. On September 26, 2010, Chavez' party, the United Socialist Party of Venezuela, took 57.4% of parliamentary seats with only 48.2% of the vote under the new system (ignoring the role of small allied parties). One can see to what extent parallel voting had nonetheless helped to redress the balance towards proportionality somewhat by noting that Chavez' party would have taken an even larger share of assembly seats under a strict single-winner approach (71 constituency seats out of 109, or 65%).
A final example is that of the Italian general election, 2001, in which one of the two main coalitions (the House of Freedoms, which opposed the scorporo system, (an alternate version of MMP), linked many of their constituency candidates to a decoy list (liste civetta) in the proportional parts, under the name Abolizione Scorporo. As a defensive move, the other coalition, Olive Tree, felt obliged to do the same, under the name Paese Nuovo. This meant that the constituency seats won by each coalition would not reduce the number of list seats available to them. In the case the House of Freedoms list faction Forza Italia, the tactic was so successful that it did not have enough candidates in the proportional part to receive as many seats as it in fact won, missing out on 12 seats.