First, a little bit of civics:
The Presidential Election is actually an amalgam of 50 separate elections held in each one of the United States. In these State-level elections, the winner is chosen by popular vote, which in turn allocates a certain number of Electors, who then go on to vote in the national election, which takes place in something called the Electoral College. The Electors are awarded based on the popular election, which is held simultaneously in each State on the first Tuesday in November. The Electors in the Electoral College then conduct their vote in mid December.
In 48 of the 50 States, Electors are awarded in a “first past the post” system, which is to say the candidate who wins the plurality of the popular vote is awarded ALL of the Electors from that State (Nebraska and Maine award Electors proportionately). The number of Electors in each State is based on the number of Representatives that the State sends to the House of Representatives, plus the two Senators that the State sends to the Senate. The District of Columbia (a.k.a. Washington, DC), while not an actual State, also has 3 Electors that participate in the presidential elections.
The reason that the US has such an arcane and unusual system stems from America’s origins as a slave-holding country. When the United States was founded, the center of economic and thus political power resided in the slave-holding colonies of the South. Two thirds of the wealth of the original American colonies was located in the slave-holding South, and slave-holders were the richest and most powerful class of people in the New World.
Indeed, the concentration of economic power that obtained at the time of independence persisted up until the time of the Civil War:
..by 1860, there were more millionaires (slaveholders all) living in the lower Mississippi Valley than anywhere else in the United States. In the same year, the nearly 4 million American slaves were worth some $3.5 billion, making them the largest single financial asset in the entire U.S. economy, worth more than all manufacturing and railroads combined.
Why is this important?
The founders of the US decreed that only white male property owners would be permitted to vote in elections. This decision, however, meant that In terms of would-be voters in the new republic, the Southern states were sparsely populated compared to the North. Yet, because the voting franchise also required that one own property, the Southern white slave holders demanded that the wealth and value of their “property” of the (i.e., the black slaves themselves) be reflected in their political power and representation in Congress.
After much argument between the Northern and Southern delegates to the Constitutional Convention, it was agreed that, for the purposes of calculating the number of Representatives to the Congress, slaves would also be counted at the rate of 3/5 of a person each. This was known as “the Three-Fifths Compromise”:
Article I, Section 2 of the U.S. Constitution states: “Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers, which shall be determined by adding to the whole Number of free Persons, including those bound to Service for a Term of Years, and excluding Indians not taxed, three fifths of all other Persons.” The “other Persons” were slaves.
This rule ensured that the wealthy Southern slave holders would have representation and power in the government that would be highly disproportionate in terms of their number, but more closely aligned in terms of their wealth. The populations of the Southern States were thus enumerated politically in numbers that far outweighed their actual number of voters. Indeed, in the Deep South, non-voting black slaves made up 70% of the actual inhabitants at the time.
This meant that a Southern white male in those States had a representational “weight” in government that was roughly 2.5 times what it should have been.
This was on average, of course. In one extreme example, as of the Census in 1790, Georgia had one Electoral College vote for every 6,000 citizens, while Pennsylvania had one EC vote for every 23,000.
This over-representation of wealth and capital in the US political system continues to this day, despite the expansion of the voting franchise to allow more people to vote in popular elections. Indeed, the recent Supreme Court decision on Citizens United (2008), which allows for unlimited spending in elections, is but the latest manifestation of the peculiarly American predilection to allow wealth to control the US political system.
And while Representatives today are assigned based on actual and total populations, the fact that each state has 2 Senators and thus receives at least 2 Electors regardless of size, and the fact that each State no matter how small must have at least one Representative, means that sparsely populated States still have an out-sized representational influence not only on the Legislative Branch of the government, but also the Executive Branch, thanks to the make-up of the Electoral College.
Below is a map of the US that indicates the population size of each State:
It should also be noted that such underpopulated and yet over-represented States still tend to be mostly white, for example Alaska, Wyoming, Montana, the Dakotas and Vermont, all of which tend to vote Republican in national elections.
To provide a modern-day example of Electoral College bias: as of the Cenus in 2010, Wyoming had one Electoral College vote for every 187,000 citizens, while California had one EC vote for every 677,000.
It is thus that Republicans have a decided advantage when it comes to presidential elections, and it is thanks to the Electoral College, that anachronistic vestige of slavery, that it has come to pass that in 2 of the past 5 presidential elections (40%), the winner of the popular vote failed to win the White House.