Two-party system

A two-party system is a form of party system where two major political parties dominate the voting in nearly all elections. As a result, all, or nearly all, elected offices end up being held by candidates endorsed by the two major parties. Coalition governments occur only rarely in two-party systems, though each party may internally look like a coalition.

Under a two-party system, one of two major parties typically holds a majority in the legislature (or a legislative house in a bicameral system), and is referred to as the Majority Party. The other major party is referred to as the Minority Party. The leader of the majority party may be referred to as the Majority Leader, assisted by the Majority Whip, and the leader of the major opposition party may be referred to as the Minority Leader, assisted by the Minority Whip.

How and why it occurs

Two-party systems often develop spontaneously when the voting system used for elections discriminates against third or smaller parties, because the number of votes received for a party in a whole country is not directly related to the proportion of seats it receives in the country's assembly/assemblies. While there is sometimes a coincidental relationship between votes cast and seats received in these systems, voters are not assured that their one vote will directly count toward an additional seat. The most widely-used system to have this effect, the simple plurality system (first past the post) often appears to pull systems into encouraging the survival of only two major parties, because it encourages compromising, where one insincerely votes for another party in the hopes that the other worse party will not be elected. It is also vulnerable to vote splitting. A third force can break in on the scene (the Labour Party in 20th century United Kingdom, or arguably the Republican Party in the 19th century United States, for example) but only at the ultimate expense of a former major party (the Liberal Party and the Whigs, respectively). The overall system re-stabilizes into two-party mode after a three-party interlude - see Duverger's law. Runoff voting can also help solidify a two party system.

Some representation systems - such as those involving a single elected president or a mayor dominating the government - may encourage two-party systems, since ultimately the contest will pit the two most popular candidates against each other.

When constituencies (electoral districts) vote for candidates on the basis of a geographical constituency, all votes for candidates other than the winner count for nothing. This reflects another factor that encourages a two party system: smaller parties often cannot win enough votes in a constituency because they have smaller support and sometimes more scattered support than larger parties. Often a first-past-the-post electoral system and the election of candidates from geographical constituencies (districts) appear together in a single political system: this means that some smaller parties can garner a significant proportion of votes nationally, but receive few constituency seats and thus cannot realistically expect to compete overall on an equal footing with larger country-wide parties.

In countries that use proportional representation (PR), especially where the whole country forms a single constituency (like Israel), the electoral rules discourage a two-party system; the number of votes received for a party relates directly and proportionally to the number of representative seats won, and new parties can thus develop an immediate electoral niche. Duverger identified that the use of proportional representation would make a two party system less likely. However, other systems do not guarantee new parties access to the system: Malta provides an example of a stable two-party system using the single transferable vote. The Dutch political system is sometimes called the most representative in the world, in part because it has no electoral threshold (only residual seat assignment disturbs the representativeness).

Often, two-party systems result from various factors, mostly the use of a first-past-the-post voting system, rather than from deliberate electoral/political engineering.

Advantages and disadvantages

The two-party system's defenders argue that:

Uncommon, unconventional ideas and ideologies remain non-influential, so policies and governments do not change rapidly. (Others dispute whether such innate conservatism provides advantages. While smaller parties find this exceptionally frustrating, proponents of the two-party system suggest that it enhances stability while eventually allowing for ideas that gain favor to become politically influential.)
The dynamics of a two-party system drives both parties' policies towards the position of a mythical median voter while remaining (hopefully) distinctive enough to motivate their core support. This "middle anchor" can work to prevent both parties from shifting too far to either side of the political spectrum.
Two-party systems, especially those where power often changes hands, appear less prone to revolutions, coups, or civil wars.
Two-party systems provide two distinct political parties that average people can choose between based on core principles and political ideology.
Two-party systems encourage unity among individuals of similar but not identical ideologies rather than encouraging segregation based on minor differences of opinion. This unity and its inherent necessity of civil compromise work to combat the dissonance that can arise in multi-party systems.
Bickering of narrowly based ideological factions in multi-party systems can lead to a torpid legislative process. These factions, if they gain enough influence via winning seats, can adopt a "by any means necessary" mentality of furthering their agenda which can include purposely blocking or delaying important legislation.
Narrowly based ideological factions can force the major parties to help them in exchange for their support. This can create a chaotic and fluctuating system of alliances that intensifies confusion among voters. Additionally, this "tie-breaker" influence minor parties achieve can serve to undermine the true positions of the major parties.
Narrowly based ideological factions can have a trickle down effect on a nation's citizens. People can become so obsessed with the single issue parties they belong to (e.g. Environmental Party) that they begin to give very little thought to other issues that are of equal or even greater importance.
Multi-party systems have fractionalized legislatures that encourage the same fractionalization among a country's electorate. Hence, encouraging segregation in government has a trickle down effect. This can be devastating in developing countries where violence is often an attractive form of negotiation. In developed countries this fractionalization can weaken national unity and patriotism, both of which are vital to the strength and defense of a sovereign nation.
The major parties are really broadly based coalitions that already represent a great diversity of views. By welcoming many different opinions and formulating positions based on those opinions, the major parties prevent themselves from becoming narrowly based ideological factions. At the same time however, as is the case in the United States, the major parties maintain an ideological and easily identifable identity. In the United States for instance, citizens do not have to be Political Scientists to identifty the ideological differences between the Republican and Democratic parties. Voters choose between the two major parties and find common ground with one another based on the party they support. Hence, encouraging unity and civil compromise rather than fractionalization in government encourages the same among the people.

Against the argument that the two-party system leads to more stable governance, critics of two-party systems argue that:

It is impossible to nominate, let alone elect, a moderate candidate. Among non-moderates (for instance, Democrats or Republicans in the US), a candidate's campaign can become extremely complicated if he chooses to break with his party's ideology on any specific issue (or concede that he does not subscribe to the beliefs of some members of his party). For this reason, many argue that the two-party system is conducive to hardliner ideology and extremist politics.
The ruling party's majority may still be based on a smaller segment of the population than coalition governments due to lower turnout, and votes cast that do not lead to the desired representative [1]. With lower voter turn-out plus only a chance of getting the representative voters want, the ultimate body of representatives were voted in with a rather small number of votes (such as 40%). The majority of this body (20% plus one) rules the nation.[2]
Stability is not desirable in itself. The two parties in power resemble each other so much on the major issues and in their wealthy power base that the two party system more resembles a one party system.
Elections based on geographical district representation can become subject to gerrymander. Even without deliberate partisan gerrymander, legislative representation can skew wildly from the actual percentage of the vote a party wins. For example, the party with the second-highest total vote tally actually won a parliamentary majority in the 1847, 1852, 1874, and 1951 UK general elections, and the most seats in the January 1910, December 1910, 1929, and February 1974 elections.
The two-party system does produce stable governments, but this comes at the expense of the preferred outcome of stable democracy.
Two-party systems do not appear intrinsically more stable, citing such examples of stable democracies as Germany, that has representation through district elections, but becomes a multi-party system through a correcting format of the overall vote.

Observers also criticise two-party systems for the following alleged flaws:

Simplified (virtual two-way) elections motivate candidates to run negative campaigns, pointing out the flaws in the "other person" (usually the leader of the other party). Parties in such situations tend to stake out only those positions that appear necessary to differentiate themselves from their primary opponent, and not to concentrate on policies constructive or beneficial to citizens.
If one of the two parties becomes weak, a dominant-party system may develop.
Debate in the assembly of the country can often become adversarial and not constructive, sometimes revolving around narrowly-perceived policy ideas, rather than larger political issues. Sometimes adversarial politics can lead to the opposition disagreeing with everything the government proposes (and vice versa) for the sake of disagreeing. This can lead to the blocking of important legislation, especially reforms that may benefit the country.
Campaign contributions can more easily corrupt a two-party system - since it has fewer players to receive donations.
In an effort to attract voters, each party will adopt planks of the other party's platform, leading to the appearance in some skeptics' minds of a one-party system. Examples include the American notion of a "Republicrat".
First-past-the-post election systems tend to produce fewer female and minority representatives than proportional representation systems.
District elections tend to deliver a larger economic gap between members within its society. The have's tend to have more and the have-not's tend to have less in these nations.^{[citation needed]}

The electoral systems which tend to favour two-party systems (notably the "biggest pile of votes wins" system) also receive criticism because:

Most electors have perforce to engage in tactical voting, voting for candidates that may not be their first choice - either to help a perceived potential "winner" or to block a potential enemy.
Smaller parties suffer from under-representation: they will not receive a number of seats in the country's assembly that reflects the number of votes they receive (and therefore the amount of support they could or do receive). Some see this as undemocratic, arguing that citizens who vote for small parties should receive fair representation.
Smaller parties often represent unconventional or 'alternative' (compared to the main parties) ideologies and formulate policy on the basis of such ideologies. Some commentators argue that in a democracy, all supported ideologies should receive fair representation.
Larger parties benefit from over-representation; some see this as undemocratic.
'Average voters' may dislike suggestions that they require a simple choice as implying they are incapable of an informed decision.