Every 4 years, the corresponding Tuesday between November 2 and 8, and after almost a year of campaigning, US citizens are ready to vote for their candidates for president. A short time later, in the month of December, the president and vice president of the United States are elected by the vote of only 538 citizens called "electors" of the Electoral College.
The president of the United States is elected in an assembly formed by 538 voters. This figure is equal to the sum of 100 senators + 435 congressmen + 3 delegates from Washington D.C., who does not have senators but delegates. Each state contributes with a block of these delegates, whose number is equal to the sum of their representatives plus their senators or delegates.
On the ballot papers, each candidate for president has the name of their vice president and the political party to which they belong. But these votes do not elect the president for the moment, but rather they choose en bloc the delegates of this political option who will go to the electoral college later. As there are 538 delegates in total, a candidate needs at least 270 to be elected. Which translates into half of those 538 or 269 + 1 = 270 delegates to be elected President of the United States.
When a citizen votes for his or her candidate for president, this person is actually voting to instruct the elector of his or her state where his or her vote should be directed in the Electoral College. For example, if a citizen deposits his ballot for the candidate of the Republican party, this person is really ordering the "elector" of his state to vote for that candidate at the meeting of voters in the electoral college, the same in the Democratic case. Or what is the same, whoever wins the popular vote in a certain state, will get the support of "the electors" and, therefore, the state votes to that candidate and his party.
If it happens that none of the candidates get more than 269 electoral votes, the Twelfth Amendment to the Constitution comes into force and Congress decides who will be the new president. The combination of congressmen from each state is entitled to one vote per state and a simple majority of states gives one winner. This situation has occurred twice in American history. The first occurred in 1801: Thomas Jefferson was elected president; the second occurred in 1825, when President John Quincy Adams was elected.
Critics of the electoral voting system emphasize the fact that a candidate for president, still losing popular elections, can obtain 270 votes and, therefore, become president by the Electoral College.