The popular vote in each of the fifty US states, rather than the total national vote, plays a role in selecting the next President of the United States. Each state's popular vote affects the outcome in the Electoral College, which ultimately determines the winner. States that have a history of normally voting Republican are called ‘red states', while those which traditionally vote Democrat are called ‘blue states'.
Source: New York Times
Battleground states, also called swing states or purple states, are the American equivalent of marginal seats in Australia. Voters in these states are so evenly divided in their political allegiances that state wide support can switch from one party to another from election to election. Every state except Maine and Nebraska awards all of their electoral votes to the candidate who wins the popular vote in the state, making swing states especially important. California may have 38 million residents, but, since it votes reliably Democratic, it does not make sense for candidates to spend a lot of time campaigning there. Regardless of whether the Democratic candidate wins 54 percent or 70 percent of the vote in the state, they will still receive all 55 of California’s electoral votes.
Consequently, Presidential candidates focus much of their campaign on winning in the battleground states. If the vote is closely contested within a state, converting a few more voters in the state is incredibly valuable. Here are the seven swing states in the 2012 election along with their number of electoral votes: Colorado (9), Florida (29), Iowa (6), Ohio (18), Nevada (6), New Hampshire (4) and Virginia (13).