Partisan system
A partisan system is a type of electoral system which uses partisan votes instead of voting for members to be elected. The most simple type of Partisan System is the party-list system. It is contrasted by a regional system which elects members without any reference to their associated party but instead grouped into regional districts. There are also mixed electoral systems where voters can vote for both a party and a regional representative.
Purpose
The justification for partisan systems is to be able to ensure maximal proportional representation. Since proportional representation is a measure which refers to the party the support for each party is needed for the calculation.
All partisan systems use plurality voting for the parties because of the express goal to use the partisan votes in proportional representation calculations. Variations often depend on which measure of proportional representation is being used.
Context
From a historical perspective, parties have been viewed as a necessary evil of the political process. George Washington spoke out against political parties in general in his 1796 Farewell Address, claiming their existence "agitates the community with ill-founded jealousies and false alarms, kindles the animosity of one part against another."
The Single Member Plurality system is not a partisan system since parties are not part of the election process by prescription and parties are not voted for directly. Even so, in most countries with Single Member Plurality parties do exist and have a great deal of power in how the country is governed. The most common argument for adopting a partisan System with explicitly partisan voting is that this is the only way to achieve optimal Proportional Representation. The argument for Proportional Representation stems from looking at politics from the standpoint of the political parties not the individual citizen. Most systems have a method of government formation which is a zero-sum game for political parties. Ensuring Proportional Representation would then make it a fair zero-sum game for the parties.
The whole conception of Representative Government is different under a partisan system. Changing to a Party List partisan system from a Single Member Plurality Regional System is a change from representation of all people by elected members, to a representation of some people by political parties through elected members. The loss of representation and fairness to the people is traded for a gain in fairness for the parties in the form of increased Proportional Representation. Imagine a scenario where there are two parties, party A gets 51% of the popular vote and 49% of the seats while party B gets the opposite. It could be argued that this is unfair to Party A because they are not the ones to form the government even though they have the popular vote. This is more of an issue with how governments are formed than the election of members to the assembly, but in the Westminster System it is a clear problem. Another scenario is that a party which gets 5% of the national popular vote could receive 0% of the seats. This party could also claim that this is unfair since the party does not get the representation it "deserves" in the assembly. A representative government is intended to represent the citizens, not the party. However, those who are strongly aligned with a party will share in the parties’ feeling of unfairness. Adoption of a Partisan System to increase Proportional Representation would be an act by those who support a specific party to disempower those who do not support any.
Non-dominant parties with broad national interest would gain seats from a change to a partisan system from a Regional System. This means that small parties tend to favour such a system. A case can be made that small partisan groups are closer to the special interest groups that one would ideally like to represent to improve Ideal Representation. In theory, this can be true but in practice there are several barriers that prevent this from working. The first and most obvious is that a group that is large enough to define a political party is generally large enough to have a representative voice on key issues in other parties. This is because the ideological boundaries between parties overlap.
That a small party represents a specific unrepresented issue is not seen often in practice. An issue which is too obscure to get attention or a group which does not have enough members to have a national voice in a Regional System would likely not have its due representation in a Partisan System either. For example, if a particular issue with technological security were to come up and only a few experts were aware of the general threat and how legislation could be formed to guard against it; how then, is a party intended to form to address this issue? The issue can gain attention in major parties through lobbies, but that necessitates monetary backing. Some issues can get media attention if they appeal to the emotions or fears of the public. Another example would be groups that desire representation due to their unique interests. These could form due to regional, occupational, cultural or any other identifiable characteristic. As stated above this is not an issue if the group is large enough to influence a major party. In a Regional System, the person who is aware of the issue is in tended to contact their representative and petition their case but this is not possible in a Partisan system where there is no specific representative. From this, it can be seen that Partisan Systems have a tendency to exacerbate the problem of unrepresented issues by giving the power to large parties which have no incentive to listen to such petitioners. Ironically, proponents of Partisan Systems often claim the opposite. This means that a gain in Proportional Representation can come at the cost of Ideal Representation from the space of opinion.
A further issue is that the whole of the ideological center who hold nuanced ideas and partially supports many parties on specific issues are not expected to be represented by anybody. In his landmark book “The Logic of Collective Action”, Mancur Olson referred to these people as “The ‘Forgetten Groups’, Those Who Suffer In Silence”.
In the presence of large parties in a partisan system, it is worth considering what a small party would represent. In some cases, it would be the desired special interest issues and groups but not in all. If it is not a single issue then it must be fringe in terms of general ideology. This is typically the far left and far right. It is unclear if political radicals would be productive members of the assembly if their views are by definition inconsistent with the majority. Some implementations of Partisan Systems require a party to win several seats to be able to claim any. This is generally argued on the grounds of preventing ethno-nationalist parties from having a voice but it clearly would eliminate any possible gain in Proportional Representation for any small group. This means that proportional representation would not help small parties so the one major argument for Partisan systems would be lost. The other argument for Proportional Representation regarding fairness in government formation is better solved by solving government formation itself. In summary, some individual citizens will lose a measure of representation from Partisan systems, but a good degree of power would be gained by political parties so the adherents of parties would gain.