Final answer:
In the US, a party that is formed as an alternative to the existing major parties in a two-party system is known as a third or minor party.
Step-by-step explanation:
In the US, a party organized as opposition or an alternative to the existing parties in a two-party system is known as a third party. These parties are also referred to as minor parties. Over the course of US history, various third parties have appeared, some with lasting impact and others that have faded away. For example, the Populist Party emerged in 1892 with a platform advocating the regulation of railroads, an income tax, and the popular election of U.S. senators. Though these parties often struggle to win national office due to the winner-take-all Electoral College system and state-level ballot access rules, they can still be influential by emphasizing issues important to voters that may be ignored by the two major parties. Contemporary third parties like the Libertarian, Green, and Constitution parties continue to offer alternative viewpoints despite the challenges posed by the two-party system in the United States.