A political party is defined as an organization that consists of individuals with a common body of principles and goals who attempt to gain control of government by winning elections. Political parties differ from interest groups in that the ultimate goal of an interest group is not to gain control of government but to influence government decisions. There is no mention of political parties in the U.S. Constitution. The founders originally had negative attitudes toward political parties.
The basic structure of the two major parties in the United States, Democratic and Republican, is coalitional. It means that each party is made-up of subgroups that band together to win elections. Parties in Europe, as well as some third parties in the U.S., like the Libertarian Party, exhibit a structure that can be labeled ideological. It means that the party is formed based on a single or a set of narrowly related set of principles to which its members are expected to adhere. For political parties that emphasize ideology, fidelity to principle is more important than winning elections.
Both the Democratic and Republican parties have the same organizational structure. The party structure is divided into permanent and temporary. The former consists of party chairs and party officers at local and state levels and the latter is composed of the primary election and conventions held at local and state level every two years. Each party has party organization at the precinct and county levels, where voters elect a precinct chair and county chair at their respective party primary election. The state party chair and members of its executive committee are elected in each party's state convention. Each party uses temporary party structure to hold its primary election and its conventions. The major function of the party convention is to adopt resolutions and elect delegates to attend the next higher party convention.