Due to the winner-take-all style of the Electoral College, candidates often campaign only in competitive states, which is why a select group of states frequently receives a majority of the advertisements and partisan media. The battlegrounds may change in certain election cycles, and may be reflected in overall polling, demographics, and the ideological appeal of the nominees. Election analytics website FiveThirtyEight identifies the states of Colorado, Florida, Iowa, Michigan, Minnesota, Ohio, Nevada, New Hampshire, North Carolina, Pennsylvania, Virginia, and Wisconsin as "perennial" swing states that have regularly seen close contests over the last few presidential campaigns.
In American presidential elections, each state is free to decide the method by which its electors to the Electoral College will be chosen. To increase its voting power in the Electoral College system, every state, with the exceptions of Maine and Nebraska, has adopted a winner-take-all system, where the candidate who wins the most popular votes in a state wins all of that state's electoral votes. The expectation was that the candidates would look after the interests of the states with the most electoral votes. However, in practice, most voters tend not to change party allegiance from one election to the next, leading presidential candidates to concentrate their limited time and resources campaigning in those states that they believe they can swing towards them or stop states from swinging away from them, and not to spend time or resources in states they expect to win or lose. Because of the electoral system, the campaigns are less concerned with increasing a candidate's national popular vote, tending instead to concentrate on the popular vote only in those states which will provide the electoral votes it needs to win the election, and it is far from unheard of for a candidate to secure sufficient electoral votes while not having won the national popular vote, such as in the case of the 2016 presidential election. From past electoral results, a Republican candidate can expect to easily win most of the mountain states and Great Plains, such as Idaho, Wyoming, the Dakotas, Montana, Utah, Kansas, Oklahoma, and Nebraska, while a Democrat usually takes the New England states, including Vermont, Massachusetts, Rhode Island, and Connecticut. However, approximately half of New England currently has a sitting Republican Governor, as well as many Mid-Atlantic states that typically vote for Democrats.
In Maine and Nebraska, the apportionment of electoral votes parallels that for Senators and Congressional Representatives. Two electoral votes go to the person who wins a plurality in the state, and a candidate gets one additional electoral vote for each Congressional District in which they receive a plurality. Both of these states have relatively few electoral votes – a total of 4 and 5, respectively. Neither Maine, which is generally considered a Democratic-leaning state, nor Nebraska, typically thought to be safely Republican, would become battlegrounds in the event of a close national race. Despite their rules, only once has each state 'split' its electoral votes – in 2008, when Nebraska gave 4 votes to Republican John McCain, and one to Democrat Barack Obama; and in 2016, when one of Maine's congressional districts was won by Donald Trump, and the other district and the state itself were won by Hillary Clinton.
States where the election has a close result become less meaningful in landslide elections. Instead, states which vote similarly to the national vote proportions are more likely to appear as the closest states. For example, the states in the 1984 election with the tightest results were Minnesota and Massachusetts. A campaign strategy centered on them, however, would not have been meaningful in the Electoral College, as Democratic nominee Walter Mondale required victories in many more states than Massachusetts, Republican Ronald Reagan still would have won by a large margin.
Instead, the closest state that year was Michigan, as it gave Reagan the decisive electoral vote. The difference in Michigan was nineteen percentage points, quite similar to Reagan's national margin of eighteen percent. Michigan would have been more relevant to the election results had the election been closer. Similarly, Barack Obama's narrow victory in Indiana in the 2008 election inaccurately portrays its status as a battleground. Obama lost Indiana by more than ten percentage points in the closer 2012 election, but triumphed despite losing less Republican states like North Carolina, Arizona, Georgia, Missouri, and Montana.