Duverger's law re the two party system and third parties in winner take all elections.....
Explanations for why a country with free elections may evolve into a two-party system have been debated. A leading theory, referred to as Duverger's law, states that two parties are a natural result of a winner-take-all voting system.
Duverger's law also explains why third party candidates have little or no chance of winning in two-party systems.
Origins and History in a Nutshell:
There is general agreement that the United States has a two-party system; historically, there have been few instances in which third party candidates won an election. In the First Party System, only Alexander Hamilton's Federalist Party and Thomas Jefferson's Democratic-Republican Party were significant political parties. Toward the end of the First Party System, the Republicans dominated a one-party system (primarily under the Presidency of James Monroe). Under the Second Party System, the Democratic-Republican Party split during the election of 1824 into Adams' Men and Jackson's Men. In 1828, the modern Democratic Party formed in support of Andrew Jackson. The National Republicans were formed in support of John Quincy Adams. After the National Republicans collapsed, the Whig Party and the Free Soil Party quickly formed and collapsed. In 1854, the modern Republican Party formed from a loose coalition of former Whigs, Free Soilers and other anti-slavery activists.Abraham Lincoln became the first Republican president in 1860.
During the Third Party System, the Republican Party was the dominant political faction, but the Democrats held a strong, loyal coalition in the Solid South. During the Fourth Party System, the Republicans remained the dominant Presidential party, although Democrats Grover Cleveland and Woodrow Wilson were both elected to two terms. In 1932, at the onset of the Fifth Party System, Democrats took firm control of national politics with the landslide victories o fFranklin D. Roosevelt in four consecutive elections. Other than the two terms of Republican Dwight Eisenhower from 1953 to 1961, Democrats retained firm control of the Presidency until the mid-1960s. Since the mid-1960s, despite a number of landslides (such as Richard Nixon carrying 49 states and 61% of the popular vote over George McGovern in 1972; Ronald Reagan carrying 49 states and 58% of the popular vote over Walter Mondale in 1984), Presidential elections have been competitive between the predominant Republican and Democratic parties and no one party has been able to hold the Presidency for more than three consecutive terms, except for the elections from 1932 through 1948 when FDR won 4 elections and Truman one, both as Democrats. In the election of 2012, only 4% separated the popular vote between Barack Obama(51%) and Mitt Romney (47%), although Obama won the electoral vote by a landslide (332–206).
Throughout every American party system, no third party has won a Presidential election or majorities in either house of Congress. Despite that, third parties and third party candidates have gained traction and support. In the election of 1912, Theodore Roosevelt won 27% of the popular vote and 88 electoral votes running as a Progressive. In the 1992 Presidential election, Ross Perot won 19% of the popular vote but no electoral votes running as an Independent.
Duverger's Law and Third Parties:
A two-party system often develops in a plurality voting system. In this system, voters have a single vote, which they can cast for a single candidate in their district, in which only one legislative seat is available. In plurality voting (i.e.first past the post), in which the winner of the seat is determined purely by the candidate with the most votes, several characteristics can serve to discourage the development of third parties and reward the two major parties....
....A third party can enter the arena only if it can exploit the mistakes of a pre-existing major party, ultimately at that party's expense.