Single-member plurality system
U.S. system requires a party to get one more vote than any other party in a legislative district or in a state’s presidential election to win. In contrast, countries that use proportional representation often guarantee parliamentary seats to any faction securing as little as 5 percent of the vote. Therefore, in the U.S., parties usually move to the left or right on issues in order to gain popular support. This tends to keep third parties remaining minor.