In an election like this, there's no guarantee that a county that went Republican or Democrat in the last election will pick the same party this year. And even if they do, the winning party might squeak by with a much smaller lead than in 2012.
A lot can change across more than 3,000 counties over the course of four years: Populations can rise or fall, demographics can shift dramatically, or residents may simply find the candidate from the opposing party more palatable this time.
CNBC's Big Crunch data team, with data from The Associated Press and NBC, is keeping track in real time of how voters across the country are changing their party support since the 2012 election. Use the map below to see the trends. This is an ongoing story, check in later for updates.
The colors in the map show the winner in each county: Blue means Clinton got more of the vote, red means Trump did. The darker shade means the winner won by a larger margin than their party did in 2012. The lighter shade means the winner did worse than in 2012; they still won, but by a smaller margin.
In this election, it's possible that Donald Trump could make inroads with blue-collar workers who have traditionally supported Democrats, or that Hillary Clinton could win some votes in traditional Republican strongholds that have growing immigrant populations. Trump has tried to appeal to disaffected Democrats who were disappointed with Sen. Bernie Sanders' loss in the primary.
Because so much of the electorate is entrenched in their own political worldview, one of the big goals for political campaigns is to persuade as many independents and swing voters as possible to join them. Flipping a few counties and even a state or two from the previous election can make the difference in a close race.
Please note: This map is not meant to show the most up-to-date election returns. You can find minute-by-minute results and projections on CNBC's election page. This map only includes data from counties and other voting subdivisions that have at least half of precincts reporting.
This map should also not be used to get a sense of which candidate is pulling ahead. Counties are sized according to land mass and not population, so large counties with them are more visible on this map even if they contribute fewer votes. The American electoral system is based on winning states, not counties, so winning additional votes in swing states like Ohio, Florida or North Carolina is more valuable than those that will almost certainly be red or blue.
Note: A small number of counties in the graphic above were colored incorrectly due to an error in the underlying data and have been corrected.