Final answer:
The Electoral College system favors a two-party system due to the winner-take-all electoral votes allocation in most states. Third parties face difficulties in this structure, whereas pluralistic or proportional representation systems promote multi-party structures.
Step-by-step explanation:
The Electoral College system in the United States tends to favor a two-party system. This system, based on the winner-take-all approach in all but two states, means that all of a state's electoral votes go to the candidate who wins a plurality of the popular vote in that state. For any third party to be successful, it must win the support of voters in several states to gain enough electoral votes. This makes it challenging for new or minor parties to have a significant impact on the presidential election outcome due to the hurdle of securing widespread, state-level support.
In contrast, systems that employ proportional representation or preferential voting, as seen in some European countries and Australia, tend to result in multi-party systems. Proportional representation allows multiple parties to earn seats in the legislature according to the percentage of votes they receive nationally, supporting a wider political representation.
Critically, while the winner-take-all nature of the Electoral College bolsters the two-party system, it is not the sole reason for its prevalence. Historical political divides and the structuring of political parties in the U.S. also deeply entrench this system, alongside rules that benefit established major parties.