Parallel voting

The supplementary member system allows smaller parties that cannot win individual elections to secure some representation in the legislature; however, unlike in a proportional system they will have a substantially smaller delegation than their share of the total vote. It is also argued that SM does not lead to the degree of fragmentation found in party systems under pure forms of proportional representation.[3]

A criticism of proportional electoral systems is that the largest parties need to rely on the support of smaller ones in order to form a government. However, smaller parties are still disadvantaged as the larger parties still predominate. In countries where there is one dominant party and a divided opposition, the proportional seats may be essential for allowing an effective opposition.

Because the vote is split between constituencies and a list, there is a chance that two classes of representatives will emerge under an SM system: with one class beholden to their electorate seat, and the other concerned only with their party.

The major critique of parallel systems is that they cannot guarantee overall proportionality. Large parties can win very large majorities, disproportionate to their percentage vote. For example, in the 2014 Hungarian election, the Fidesz/KDNP grouping won 133 of 199 Parliamentary seats with 44.87% of the overall vote. Small parties may still be shut out of representation despite winning a substantial portion of the overall vote.[4] An example of this being played out in reverse can be seen in the 2014 Japanese election where the government's junior coalition partner, Komeito took only 1.5% in the local constituencies, but 13.7% on the PR list. Most of the Komeito votes came from the ruling Liberal Democratic Party.

Parallel systems are often contrasted with Mixed Member systems. There are a unique set of advantages and disadvantages that apply to this specific comparison.

A party that can gerrymander local districts can win more than its share of seats. So parallel systems need fair criteria to draw district boundaries. (Under mixed member proportional representation a gerrymander can help a local candidate, but it cannot raise a major party’s share of seats.)

Countries like Japan, Russia, and Thailand adopted a parallel system to provide incentives for greater party cohesiveness. The party is sure to elect the candidates at the top of its list, guaranteeing safe seats for the leadership. By contrast, under the MMP system a party that does well in the local seats will not need or receive any compensatory list seats, so the leadership has to run in the local seats.

Parallel systems support the creation of single-party majorities more often than mixed member systems, this may be a positive or a negative depending on the view of the voter.

Parallel voting is primarily used in Asian and some of the European states.[5]

• Albania used parallel voting in the 1996 and 1997 elections (before switching to mixed-member proportional representation from 2001 to 2005).[11][12]
• Armenia
• Azerbaijan
• Bulgaria (1990, 2009)
• Croatia (1993–2001)
• Italy (1993–2005, with modifications)
• Palestinian Authority (2005), for the next election, the system was changed to party-list proportional representation.
• Republic of Macedonia (1998)
• Santiago del Estero Province, Argentina (1997-2009)
• Thailand used this system previously, but changed to a variant of mixed-member proportional representation for the 2019 election.[13][14]
• Egypt (2019)
• South Korea's National Assembly used parallel voting from 1988 to 2019. Since 2019, it has used a hybrid system of parallel voting and mixed-member proportional, with both compensatory seats (30) and supplementary seats (17).

Azerbaijan's National Assembly (the Milli Məclis) had previously used an SM system in which 100 members were elected for five-year terms in single-seat constituencies and 25 were members were elected by proportional representation. Since the latest election Azerbaijan has returned to electing members from single-member constituencies. Due to the corruption present within Azerbaijan,[15] the limited proportionality that SM was able to offer had little effect.

Georgia's Parliament has 150 members, known as deputies, from which 120 members are proportional representatives and 30 are elected through a single-member district plurality system to represent their constituencies. Five parties and electoral blocs had representatives elected to the parliament in the 2008 elections: the United National Movement (governing party), The Joint Opposition, the Christian-Democrats, the Labour Party and Republican Party. Due to the large amount of support given to the ruling party the disproportionality of the 2008 election was very low (1.32 on the Gallagher Index).

Starting with the 2018 election, both houses of the Italian parliament are elected using a form of parallel voting. 62.5% of the seats are assigned proportionally to party lists; party lists are also linked in coalitions supporting constituency candidates running for the remaining 37.5% of the available seats, who are elected by means of a first-past-the-post system. Electors have a single vote with two-fold proportional effects for a party list and its associated coalition candidate (split-ticket voting is not allowed).

The Kazakhstan Parliament has two chambers: the Assembly and the Senate. The Assembly (Mazhilis) has 77 seats, 67 of these are constituency seats and 10 list seats determined by proportional representation.

The Royal Commission on the Electoral System reviewed the New Zealand electoral system in 1985-86 and considered SM to be a possible replacement for plurality voting, which was in use at the time. They suggested SM could be implemented in New Zealand with the following features: each elector would have 2 votes, 1 for a constituency candidate and the other for a party list; there would be a total of 120 seats, with 90 seats determined by votes in constituencies and the remaining 30 from party lists; a modified Sainte-Laguë method would be used to allocate list seats proportionate to a party's total share of votes, a threshold of 5% was suggested before parties could be allocated seats.[16]

The commission came to the conclusion that SM would be unable to overcome the shortcomings of New Zealand’s previous plurality electoral system (FPP). The total seats won by a party would likely remain out of proportion to its share of votes—there would be a “considerable imbalance between share of the votes and share of the total seats”—and would be unfair to minor parties (who struggle to win constituency seats).[16] In the indicative 1992 electoral referendum, SM was one of the four choices of alternative electoral system (alongside MMP, AV and STV), but came last with only 5.5 percent of the vote. By clear majority, a change to MMP was favoured, as recommended by the Royal Commission, and was subsequently adopted after the 1993 electoral referendum.

In another referendum in 2011, 57.77% of voters elected to keep current the MMP system. Among the 42.23% that voted to change to another system, a plurality (46.66%) preferred a return to the pre-1994 plurality electoral system (also known as First-past-the-post, FPTP). Supplementary member was the second-most popular choice, with 24.14% of the vote.