TWO-PARTY SYSTEMS

1. Popular vote 

In the 7 presidential elections between 1992 and 2016, the Democrats and Republicans accounted for more than 80% of the popular vote on every occasion. 

2. Seats in the legislature 

Following the 2016 elections, only two members of the Senate - Bernie Sanders of Vermont and Angus King of Maine - had not been elected as either Democrats or Republicans. 

Sanders is the longest serving independent member of Congress, having been elected to the House of Representatives in 1990 and then to the Senate in 2006. 

King is a former independent governor of Maine and caucuses with the Democrats. 

3. Control of the executive.

In the White House, every president since 1853 has been either a Democrat or a Republican. 

That is 42 straight presidential elections won by the two major parties over more than a century-and-a-half. 

4. State government.

In January 2017, 49 of the 50 state governors were either Democrats or Republicans. 

1. Electoral System.

The FPTP electoral system makes life difficult for national third parties. They pick up a fraction of the vote in almost every state, but under a winner-take-all system, they receive no reward at all. 

A national third-party candidate on the ticket merely lowers the percentage of the vote needed by the major-party candidate to win the election.

2. Broad party ideologies.

When the two major parties encompass such a wide ideological spectrum, there is not much room left for any other parties to attract substantial support. 

3. Primary Elections.

The phenomenon of primary elections helps to make the major parties more responsive to the electorate, minimising the need for protest voting. 

Protest votes often go to third parties. 

The best-known national third parties are the Libertarian Party and the Green Party. 

The Libertarian Party presidential candidate Gary Johnson was on the ballot in all 50 states in the 2016 presidential election, while Green Party candidate Jill Stein was on the ballot in 44 states and was a write-in candidate in three more. 

The Green Party and the Libertarian Party are examples of permanent third parties, while the Reform Party and the American Independent Party are examples of temporary third parties. 

The Green Party and thte Prohibition Party are bot examples of issue-based third parties

The Socialist Party and the Libertarian Party are examples of ideological third parties. 

1. Ballot Access Laws

Third parties are disadvantaged by the states' ballot access laws. 

Laws in each state regulate how third-party candidates can qualify to get their name on the ballot. Some, such as those in Tennessee, are straightforward. Tennessee requires just 25 signatures on the petition, but other states, such as New York and California, demand that a third party candidate must gain a certain number of signatures in every county in the state. 

In California, the number of signatures required is equal to 1% of the electorate in the state. 

2. Lack of resources

People are reluctant to give money to parties that they know are going to lose. Third parties cannot compete with the two main parties in tems of expenditure - on organisation, staff or media.