Party system

A party system is a concept in comparative political science concerning the system of government by political parties in a democratic country. The idea is that political parties have basic similarities: they control the government, have a stable base of mass popular support, and create internal mechanisms for controlling funding, information and nominations.
The party system concept was originated by European scholars studying the United States, especially James Bryce, Giovanni Sartori and Moisey Ostrogorsky, and has been expanded to cover other democracies. Party systems can be distinguished by the degree of political fragmentation, proportionality of seats-to-votes ratio and barriers to entry to the political competition.

Types of party systems

Main classification of party systems is using the number of parties. Counting the "effective number" of parties is somewhat tricky since the decisions need to be made as to which parties shall be included into the count. Including all parties usually makes little sense: for example, in the 2005 United Kingdom general election 16 entities run candidates and 12 got seats in the parliament, however, no researcher would argue that UK at the time had a 16- or 12-party system. The practical choice would be between a two-party, or three-party system. Some researchers suggest to exclude parties with low percentage of votes, others, like Giovanni Sartori, suggest looking at the history of participation in the governments. The 2005 UK example will yield 3 parties if Ware's definition is used and 2 parties if Sartori's definition is chosen, since the Liberal Democrats almost never influenced the government formation.
The classification is based on the typology originally proposed by Jean Blondel :

One-party system: a system in which a single political party has the right to form the government, usually based on the existing constitution, or where only one party has the exclusive control over political power. Example: China
Dominant-party system: a system where there is "a category of parties/political organizations that have successively won election victories and whose future defeat cannot be envisaged or is unlikely for the foreseeable future". Example: Russia
Two party system: a system where only two parties or alliances, typically placed either side of the center, have a realistic chance of forming a majority. Other parties are very minor or solely regional. Example: United States, United Kingdom
Two-and-a-half party system where each of the two major political parties that stand apart on the political spectrum needs a coalition with a smaller "half" party for political control. Example: post-war Federal Republic of Germany until the end of the 20th century
Multi-party system: a system in which multiple political parties have the capacity to gain control of government offices, separately or in coalition. Example: Sweden, Ireland
Non-partisan system: a system of government or organization such that universal and periodic elections take place without reference to political parties. Example: Micronesia

Sartori splits the original Blondel's "one-party" category into true one party, "hegemonic", and "predominant", where competition exists, but one party on a regular basis gets over 50% of the votes. He had also split the multiparty system into "moderate pluralism" and "extreme pluralism" and introduced an "atomized" party system, where the political system is so fragmented that adding one more party does not affect the political process at all. The functioning of the moderate pluralism resembles the two-party rule: there are two camps separated in the political spectrum with established electorate, the competition occurs for the voters in the political center, the political forces are "centripetal". The "polarized pluralism" is different: "anti-system parties" position themselves at the fringes of the spectrum are detached from the center, so the parties in the center are left without a credible election threat. This results in deep political divisions, "centrifugal forces", and "irresponsible oppositions" that do "outbidding" secure in their knowledge that they will not have to govern and thus can safely over-promise. Sartori declares that the 5-party threshold between moderate and extreme pluralism is not a cause of change, but a result of the process of elite accommodation in the moderate case and lack thereof in the extreme pluralism.

Party systems by country or region

Europe

Democratic party systems in most European states have increasingly fragmented over time. That means that the number of relevant parties surged, while the average size declined. Hence, the effective number of parties increased.

European Union

The European Parliament has compared to other parliaments a higher number of political parties with 206; to reduce political fragmentation the parties are organized into 7 political groups. Two structures of party system have been identified in the European Parliament since its first universal direct election in 1979, albeit the main EU party groups remained the same:

1979–1994: a system split in two blocs on the left/right dimension, with the left bloc opposing a right bloc
1994-onwards: a system in which the three central parties have voted as much with each other as with their smaller allies, thus 'governing' the system, and facing different oppositions from the left and from the right.
Italy

Italian party systems are usually considered only since the foundation of the Italian Republic as pre-fascist parties lacked a wide popular base.
The party system of the so-called First Republic, though based on a proportional electoral law, saw the dominance of the Christian Democracy and the conventio ad excludendum against the Italian Communist Party. DC and PCI together gathered around 85% of the votes on average. The system was thus a blocked bipolar system; governments were very short and post-electoral, but the supporting parties and personnel could not change.
With time, some parties gained momentum, until reaching the role of government-making in the 1980s. The system was completely destroyed by the bribery scandals of Tangentopoli, which shattered DC and PSI.
According to Sartori, the two possible degenerations of proportionalism were reduced by two factors: the strong role of parties and the polarization between Christian-democrats and communists. Therefore, the first republic saw a maximum level of 5 effective parties, with only one dominant party.
The so-called Second Republic party system bears the following characteristic marks:

a majoritarian electoral law, introduced by referendum in 1993, which brought about a bi-polarization of the game
the birth of Forza Italia as personal party of Silvio Berlusconi, with a strong polarization effect
the rise of new parties
the split of old parties

Though more fragmented in the number of parties, the system was bipolar in its functioning. With time, both sides saw a strengthening of coalitions and the birth of unified parties.
The change in the electoral law in 2005 and the return to proportionality did not bring about a return to collusion, while still leaving such prospect open for the future.

Germany

The 2009 Bundestag election in Germany was characterized by widespread public apathy and record low voter turnout. Weldon and Nüsser argue that it solidified a new stable, but fluid five-party system that they see as a defining feature of the emerging German political system. The three minor parties each achieved historical bests at the polls with steep losses for the two traditional Volksparteien. They report that the increased volatility and fluidity of the party system is structured along the left-right ideological spectrum with the parties divided into two major camps and vote-switching much more likely within the respective camps rather than between them.
The 2009 election also marked a devastating defeat for the SPD, leading some commentators to speculate about the end of the Social Democratic Party of Germany as a "catch-all party" and, against the backdrop of recent poor performance of center-left parties all across Europe—perhaps even "the end of social democracy".
The 2013 election saw the first time that the liberal Free Democratic Party that had been represented in parliament since 1949 and formed part of government as a coalition partner to either SPD or CDU for almost all of the period from 1949 to 1998 and again from 2009 to 2013 fell below the 5% threshold for parliamentary representation. The same election also saw the rise of the "Alternative for Germany" party that ran on an anti-Euro platform and failed to enter parliament on their first federal election just barely with 4.8% of the vote.
After this election the second grand coalition between CDU and SPD since 2005 was formed. Prior to that Germany had only had one grand coalition that governed from 1966 to 1969, typically coalitions of one big and one small party at the federal level were used instead in a two-and-a-half party arrangement. Whether this shift proves temporary or permanent remains yet to be seen

Central and Eastern Europe

Four party systems have been identified in post-communist countries of Central-Eastern Europe:

I system : dominated by the opposition between communists and anti-communists, i.e. from supporters and opponents of the old regime; spontaneous mass movements formed on idealistic bases and transformed into 'umbrella parties'
II system : opposition between winners and losers of the economic transition to a market economy. Anti-communist parties split and formed unstable coalition governments. Many parties, with a narrow political base, grew up
III system : the social conflicts of market transition aggravated, and social-democratic post-communist parties took over. The party system concentrated, while electoral volatility was extremely high
IV system : rise of a relatively stable and modestly concentrated party system, organized on a left-right dimension, including post-communist parties. Fragmentation did not rise again after the fall of many social-democratic parties from government.