The correct answer is:
Third parties.
Explanation:
In the United States, there are two major political parties, the Democratic party and the Republican party. Every other party is called minor party or third party (when they are the third party with most support); through the history of the United States there have been other political parties but the ones with most support are the Democratic and Republican parties.
Third parties have obtained local and state offices, but the federal offices are not reached through this parties, although they influence elections and awareness on social problems. An example of third parties in the history of the United States are the Populist Party, the Bull Moose Party, the Socialist Party, the Green Party, the Libertarian Party and the Constitution Party.
Minor parties are usually focused on one subject and apply pressure to the major parties to focus on that subject.