To win a presidential election, a candidate must win an absolute majority of the Electoral College. The total number of electors in the Electoral College is equal to the total number of members in the Senate and House, so the winning candidate must have at least 270 electoral votes.
Population determines how many representatives each state has in the House and, therefore, how many electors it has in the Electoral College. So, different states have different numbers of votes in the Electoral College. California, for instance, has 55 electoral votes and Vermont has 3.
Every 10 years, the US government conducts its census, which, among other things, reports on changes in the US population. The census defines which states lose and gain seats in the House of Representatives. Accordingly, the census also dictates how many electoral votes each state gets.

Look at these maps from the 2008 and 2012 presidential elections. In the space provided, note the differences and similarities between the two maps. Are certain regions more Democratic and some more Republican? What other interesting points do you recognize about the maps?