http://www.thisnation.com/question/051.html

Why Do We Elect Presidents This Way?

There are two primary reasons the Founders chose to select presidents via the electoral college instead of by direct, popular voting. The first reason was their lack of trust in the judgment of the people. They were fearful that a well-spoken but not well-intentioned individual could flatter the people and win their support. They hoped that a secondary body, such as the Electoral College, would not be susceptible to such attempts at manipulation. This reason for the Electoral College is virtually meaningless with changes that have "bound" electors to cast their votes for the candidate who wins the majority of the popular vote in each state.

The second reason for choosing presidents by electoral votes instead of by popular vote is to give the states a voice in the presidential election. The principle of federalism was and is a critical feature of the American political system. By placing states in this important position in the selection of the country's leader, the Framers sought to maintain the position of states as important entities in the American political system.

Imagine that presidential elections were not decided state-by-state, but rather by a nationwide popular vote. The significance of states and the candidates' competition for support in key, "swing" states would disappear. Candidates would be inclined to simply run nationwide ad campaigns and visit large population centers.