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.

explanation below

Explanation:

The Electoral College, an assembly of the electors, is a known unique way of indirectly electing the person that would be the president of the United States of America. This college was rightly put in place through the Article 2, section 1, clause 2 of the constitution of the United States of America.  

The collage contains 538 members, one for each United States representative and senator and another three electors that represents the area known as the District of Columbia.  

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the constitution gives congress the power to regulate commerce "among" the several states.