Why does the US have a 2-Party System? What institutional factors support a 2 Party system and prevent 3rd parties?

he two-party system came into being because the structure of U.S. elections, with one seat tied to a geographic district, tends to lead to dominance by two major political parties

Explanation:Even when there are other options on the ballot, most voters understand that minor parties have no real chance of winning even a single office. Hence, they vote for candidates of the two major parties in order to support a potential winner. Of the 535 members of the House and Senate, only a handful identify as something other than Republican or Democrat. Third parties have fared no better in presidential elections. No third-party candidate has ever won the presidency. Some historians or political scientists might consider Abraham Lincoln to have been such a candidate, but in 1860, the Republicans were a major party that had subsumed members of earlier parties, such as the Whig Party, and they were the only major party other than the Democratic Party.

b) the importance and limits of political power.

troy donahue,   i think.