Though many citizens of the United States are under the impression that the national popular vote every four years leads directly to the election of the President, but in reality the popular vote simply elects electors to the Electoral College, which then votes for the President. Almost all states (Nebraska and Maine are the only two exceptions) have a winner-takes-all rule, whereby the winner of the popular vote receives all of that state's electoral votes. The number of electors from each state is dependent on that state's population, and this means that the winner-takes-all rule can have major effects on the outcome of the Electoral College's vote. California, for example, has 55 electoral votes, and even if the popular vote is split 51% to 49%, all 55 of these votes will go to the candidate that received 51% of the popular vote, effectively rendering the votes for the candidate with 49% ineffective, and over-empowering the 51% of votes for the other candidate.
This system also has a major effect on the way candidates form the two major political parties in the United States run their campaigns. In states where one party or the other can be fairly certain of a simple majority in the popular votes, neither party is likely to spend a great deal of time, money, or attention on the state. California again serves as a useful example; despite the fact that it is the most populous state in the Union, campaigns for presidential elections do not tend to be especially fierce in the state as the electoral votes from the state consistently go to the candidate from the Democratic Party. Unless the Republican Party truly thought they could win more than half of the popular votes, energy spent here is largely wasted, and the same is true for the Democratic Party who is already assured a victory. Campaigns in so-called "swing" states, where victory margins are much slimmer and voting patterns less consistent, tend to be much fiercer than in stronghold states.
The winner-takes-all aspect of the electoral college votes also greatly inhibits third party candidates by reducing their visibility in the race, and offering them absolutely no chance of election in most cases. Even if a sizable minority of voters chose Ralph Nader, a quadrennial candidate for the Presidency, as their preferred executive officer, he would receive no electoral votes in winner-take-all states. This means that even if he were to win sizeable minorities in every state, he would receive no (or almost no, thanks to Nebraska and Maine) "real" (electoral college) votes for the office, preventing him from even influencing the race between the two candidates from the major oppositional parties. The knowledge of this also reduces media and thus public attention of third party candidates during the campaign.
One of the primary reasons that the electoral college has not been abolished is that its formation and function are written into the Constitution, which is highly difficult to amend -- it has only happened seventeen times since the original ten amendments of the Bill of Rights were passed immediately following the Constitutional Convention. The laborious process of ratifying an amendment has kept the Electoral College in place largely by dissuading even any serious proposal and momentum for such an amendment even before it reaches the phase of ratification by the state legislatures. A more cynical perspective might suggest that the electoral college was devised as…