cal landscape. There has been speculation that a two-party system arose in the United States from early political battling between the federalists and anti-federalists in the first few decades after the ratification of the Constitution, according to several views. In addition, there has been more speculation that the winner-takes-all electoral system as well as particular state and federal laws regarding voting procedures helped to cause a two-party system.
Political scientists such as Maurice Duverger and William H. Riker claim that there are strong correlations between voting rules and type of party system. Jeffrey D. Sachs agreed that there was a link between voting arrangements and the effective number of parties. Sachs explained how the First Past The Post voting arrangement tended to promote a two-party system:
The main reason for America's majoritarian character is the electoral system for Congress. Members of Congress are elected in single-member districts according to the "first-past-the-post" (FPTP) principle, meaning that the candidate with the plurality of votes is the winner of the congressional seat. The losing party or parties win no representation at all. The first-past-the-post election tends to produce a small number of major parties, perhaps just two, a principle known in political science as Duverger's Law. Smaller parties are trampled in first-past-the-post elections.—Sachs, The Price of Civilization, 2011
Consider a system in which voters can vote for any candidate from any one of many parties. Suppose further that if a party gets 15% of votes, then that party will win 15% of the seats in the legislature. This is termed proportional representation or more accurately as party-proportional representation. Political scientists speculate that proportional representation leads logically to multi-party systems, since it allows new parties to build a niche in the legislature:
Because even a minor party may still obtain at least a few seats in the legislature, smaller parties have a greater incentive to organize under such electoral systems than they do in the United States.—Schmidt, Shelley, Bardes (2008), 
In contrast, a voting system that allows only a single winner for each possible legislative seat is sometimes termed a plurality voting system or single-winner voting system and is usually described under the heading of a winner–takes–all arrangement. Each voter can cast a single vote for any candidate within any given legislative district, but the candidate with the most votes wins the seat, although variants, such as requiring a majority, are sometimes used. What happens is that in a general election, a party that consistently comes in third in every district is unlikely to win any legislative seats even if there is a significant proportion of the electorate favoring its positions. This arrangement strongly favors large and well–organized political parties that are able to appeal to voters in many districts and hence win many seats, and discourages smaller or regional parties. Politically oriented people consider their only realistic way to capture political power is to be either a Republican or Democrat.
In the U.S., forty-eight states have a standard winner-takes-all electoral system for amassing presidential votes in the Electoral College system. The winner–takes–all principle applies in presidential elections, since if a presidential candidate gets the most votes in any particular state, all of the electoral votes from that state are awarded. In all but two states, Maine and Nebraska, the presidential candidate winning a plurality of votes wins all of the electoral votes, a practice called the unit rule.
Duverger concluded that "plurality election single-ballot procedures are likely to produce two-party systems, whereas proportional representation and runoff designs encourage multipartyism." He suggested there were two reasons why winner–takes–all systems leads to a two-party system. First, the weaker parties are pressured to form an alliance, sometimes called a fusion, to try to become big enough