Duverger's Law: The first-past-the-post (plurality) election system naturally leads to a two-party system. It acts to delay the emergence of a new political force, and accelerates the elimination of a weakening force, where proportional representation would have the opposite effect.
en.wikipedia.org/wiki/Duverger's_law

The plurality system of voting we use in the U.S. causes a "spoiler effect" because voters who favor a third-party candidate worry that if they vote for their preferred candidate they may actually be helping to elect a different candidate they strongly dislike, thus making third-party candidates "spoilers." Instead, voters often vote for their second or third choice candidate, known as "strategic voting" or "choosing the lesser of two evils."

Good voting systems elect the candidate most favored by the most voters while allowing voters to vote their true preference and eliminating the need to vote strategically.

Voting systems such as plurality do not meet this basic standard. They fail one of the most widely accepted and critical tests for voting systems, known as monotonicity, which states: with the relative order or rating of the other candidates unchanged, voting a candidate higher should never cause the candidate to lose, nor should voting a candidate lower ever cause the candidate to win.
electionmethods.org/evaluation.htm

Preferential voting systems, such as Ranked Pairs, remove the spoiler effect completely, and make every vote count no matter who it is cast for. If such a system were implemented in the U.S. it would allow alternative parties to compete in national elections. Variations of such systems are used successfully by democracies throughout the world. For more information on this important structural reform check out:

www.barnsdle.demon.co.uk/vote/vote.html
www.condorcet.org/rp/index.shtml
www.electionmethods.org
www.umich.edu/~cses/
www.fairvote.org
www.cfer.org/
accuratedemocracy.com/voting_rules.htm
worldpolicy.org/globalrigh...ble-pr.html