The outcome of the presidential election is determined by the Electoral College. But in many cases, it comes down to the ballots cast in just a handful of the country's more than 3,000 counties.
We've put together a cheat sheet listing the counties to watch in this year's key battleground states.
To compile the list, we asked nearly two dozen state- and national-level political experts for either longstanding bellwether counties -- those that tend to swing from year to year -- or ones that are experiencing demographic or ideological shifts that will impact the 2016 presidential election. (Some sources asked not to be identified.)
In alphabetical order by state, the counties are:
One political observer in the state calls Jefferson County "one of the most important counties in the nation and symbolic in every way of the battle for the soul of the middle class. As 'Jeffco' goes, so goes the state, and probably, the country." Voters from this older suburb that lines the western edge of Denver backed George W. Bush in 2000 and 2004, then supported Barack Obama in 2008 and 2012. More recently, voters there ousted three conservative school board members in an election that drew more than $1 million from local and national donors.
Arapahoe County is another Denver-area county and includes the large suburb of Aurora. Formerly Republican, it has morphed into a purple battleground due to a large number of immigrants who have moved there in recent years. There are 120 languages spoken in the Aurora Public Schools. Obama beat Mitt Romney by double digits in the county in 2012. At the same time, Arapahoe still has many working-class and middle-class whites and veterans, creating a measure of social and political friction.
Pueblo County, in southern Colorado, is a steel town and a union bastion. But as with other blue-collar Democratic places, it has recently begun to lean more conservative. Home to the so-called Three Plumbers, who successfully mounted recall efforts against several lawmakers for supporting gun control legislation, the county's working-class demographics could make it an area where Donald Trump does well.
Hillsborough County falls within the Interstate 4 corridor that links Tampa Bay and Orlando. "It's the most predictive swing county in the largest swing state," said University of Central Florida political scientist Aubrey Jewett. Except for 1992, Hillsborough County has backed the winning presidential candidate all the way back to 1928. It has also picked the overall winner in every statewide vote since 1996. And the margins have tended to track the statewide vote closely. "Hillsborough has been growing and becoming more diverse, thus it is also an excellent demographic reflection of Florida and to some degree the United States as a whole," said Jewett.
Pinellas County, which includes St. Petersburg, is somewhat less swingy than Hillsborough, but it's still competitive territory. It has voted for the Democratic nominee for president since 1992, except for 2004 when the county went narrowly for George W. Bush. Registration has been trending Democratic for several election cycles, but there are indications this year that the GOP may be regaining some of its strength.
Volusia County, which includes Daytona Beach, has historically leaned Democratic, but Romney won it narrowly in 2012. "No place demonstrates our struggles with working-class white voters as clearly as Volusia County," said Democratic strategist Steve Schale, who added that Romney's victory there "should be an alarm bell for Democrats." The Democrats' best hope for returning it to their column is to leverage the growing Hispanic population, primarily of Mexican and Puerto Rican descent.
Similarly, Seminole County, in the Orlando suburbs, moved somewhat toward the GOP in 2012. "It is a place where Republicans will try to run up the score to counterbalance Orlando, and a place where Democrats think they can blunt it," said Schale. Seminole has seen significant Hispanic growth. "In a few cycles, I could see this being the new bellwether county in Florida," he added.
"With its massive demographic churn, there's no place else in Georgia quite like Gwinnett County," said veteran Georgia political journalist Tom Baxter. Gwinnett, which is northeast of Atlanta, is emblematic of counties in Georgia (and elsewhere) where moderate, affluent, college-educated Republicans -- especially women -- could defect from Trump.
If Hillary Clinton is to win Georgia -- something no Democratic presidential nominee has done since her husband in 1992 -- she will also have to pump up African-American support in places like Clayton County (Atlanta), Chatham County (Savannah) and Muscogee County (Columbus).
In Iowa, there are 99 counties, but just 10 of them make up about half of the electorate in a general election. And of those 10, Polk County, which includes the state capital of Des Moines and has 14 percent of all active registered voters, carries the most weight. Obama won the county by 14 points in 2012, and Clinton will have to keep up those numbers to offset Trump's strength in more rural areas of the state. "If a Democrat doesn't win Polk County by double digits, or at least the high single digits, the rest of the state doesn't bode well," said Christopher W. Larimer, a University of Northern Iowa political scientist.
Another county to watch is Scott County, based in the Quad Cities area along the Mississippi River. It's the most evenly balanced metro-based county in the state: Six of Iowa's 10 metro counties lean Democratic and three lean Republican. Obama won Scott in 2012 by 14 points, but Republicans like Gov. Terry Branstad have done well there, and the blue-collar tinge could mean greater-than-average strength for Trump.
On a much smaller scale, keep an eye on Cedar County, a lightly populated region located between the more populous Cedar Rapids and Davenport areas. The county has received attention ever since Al Gore and George W. Bush tied there in 2000, with Gore narrowly winning on a recount. Cedar has gone for the popular-vote winner every four years since 1992, and observers say the results usually align with the statewide outcome.
Oakland County, in the suburbs of Detroit, is the state's second-most populous. With its high incomes and recent Democratic lean -- Romney lost to Obama by eight points in 2012 -- the county could be a place where Clinton does well due to favorable demographics.
By contrast, another Detroit suburb -- Macomb County, the state's third largest -- could be more promising territory for Trump. The fabled epicenter of the working-class "Reagan Democrats" went for Obama narrowly in 2012, but a Republican, George W. Bush, won it as recently as 2004.
Beyond the Detroit area, keep an eye on Ottawa County, west of Grand Rapids. It has a heavy Dutch conservative legacy, but an influx of Mexican-Americans has helped rein in Trump's prospects in this part of the state.
And Marquette County, the Upper Peninsula's biggest county, is historically Democratic, but it's overwhelmingly white and rural, making it a good spot for inroads by Trump.
Missouri has for a century been the nation's ultimate bellwether state in presidential elections. That is, until it turned Republican during Obama's two victories. Still, it's been somewhat more competitive in 2016, making three counties worth keeping tabs on.
St. Louis County is the biggest in the state, with about 19 percent of the vote. Democrats have to carry St. Louis County with 57 percent or more to win the state, said Ken Warren, a Saint Louis University political scientist.
St. Charles County, also in the St. Louis metro area, is the largest Republican county in the state. To win a state election, Republicans need to win about 56 percent of the vote in St. Charles.
Similarly, in Greene County in southwestern Missouri, Republicans need to get around 60 percent of the vote to win the state, according to Warren.
Nevada has only 16 counties, of which two are easily the most important.
Populous Clark County, which includes Las Vegas, is solidly Democratic and 29 percent Hispanic. The question for Democrats is whether turnout will be high enough to boost their chances in the state.
The other county is Reno-based Washoe County, which is a much more competitive area politically -- Obama won in 2012 with just 51 percent of the vote.
The Granite State has only 10 counties, but two have been particularly competitive in recent history: Hillsborough, which includes Manchester and Nashua, and Rockingham, which includes Portsmouth. They are the state's most populous, and they have flip-flopped regularly in presidential support -- usually by close margins.
Wake County, an urban-suburban area in the state's Research Triangle, has more voters than any other North Carolina county and is increasingly diverse. Originally Republican-leaning, it has become Democratic-leaning -- Obama won in 2012 by 11 points. "The winning formula for any Democrat in North Carolina is a big turnout and a strong percentage of the vote in Wake County," said one political observer in the state. A political journalist in the state added that "if the Trump campaign has a problem with GOP women, this is where it's going to show up."
Like Wake, Mecklenburg County, home to Charlotte, is a large urban-surburban region. When Democratic turnout was weak in 2014, Democrats did poorly in the state. "A big Democratic turnout here, plus weak support from Republicans, could be a sea anchor that drags down Trump and other statewide Republican candidates," said the journalist.
Far smaller in population than Wake and Mecklenburg -- with only 45,000-odd registered voters -- is Watauga County in the western part of the state. It's rural and mountainous but also home to a university -- Appalachian State -- and the high-end tourist community of Blowing Rock, making it a microcosm of the state's urban-rural and young-old divides. It's one of just four counties in the state with more unaffiliated voters than either Republicans or Democrats, and it has been a hotbed of controversy over election rules.
In Ohio, there's no shortage of counties to watch. That said, we narrowed it down to a handful.
Specifically, Trump could do well in these historically Democratic manufacturing and mining areas: Mahoning County (Youngstown), Stark County (Canton) and Trumbull County (Warren).
But those gains for Trump could be outweighed by Republican defections in more educated and affluent areas.
A good indicator would be fast-growing Delaware County in suburban Columbus, where Romney won 69 percent of the vote in 2012. It has voted Republican all the way back to the 1920s, but it was among Trump's worst-performing counties in the GOP primary, said Kyle Kondik, a University of Virginia political analyst and author of the book The Bellwether: Why Ohio Picks the President.
Delaware and Trumbull "will cast about an equal number of votes in the election, about 100,000, and they are exact opposites in some ways," said Kondik. "I'm curious how much Trump loses off Romney in Delaware and how much he adds in Trumbull."
On a smaller scale, Athens County, the home to Ohio University, voted by a more than 2-to-1 margin for Obama in 2012. But Clinton could see her vote tally diluted by support for third-party candidates Gary Johnson or Jill Stein.
Like Ohio, Pennsylvania has a surplus of counties to watch, particularly in two areas: blue-collar western Pennsylvania and suburban southeastern Pennsylvania.
In the latter category, four counties in the Philadelphia suburbs -- Bucks, Montgomery, Delaware and Chester -- stand out. Historically these counties are Republican, but they have trended Democratic in recent election cycles. Their income and education levels are a poor demographic fit for Trump.
Trump needs strong showings in blue-collar counties in the western part of the state, such as Beaver, Greene, Washington and Westmoreland. "They all went for Romney," said Terry Madonna, a political scientist at Franklin & Marshall University. "[But] Trump needs to exceed Romney there."
Two other counties that Trump could contest are in the northeastern part of the state: He could do unusually well for a Republican in Lackawanna County (Scranton) and Luzerne County (Wilkes-Barre), even though both Clinton and Vice President Joe Biden have roots in Scranton.
Virginia has three urban-suburban regions that form what's sometimes called the Urban Crescent. There are two counties and one city to watch in these three regions, which account for a good proportion of the votes cast in 2012.
In exurban Washington, D.C., it's Loudoun County, which was won by George W. Bush and then by Obama twice. It's home to both a growing minority population and blue-collar white voters.
Near Richmond, it's Henrico County, another historically Republican area that has diversified and voted for Obama by 12 points in 2012.
And, finally, it is the city of Chesapeake in the Hampton Roads area.
"It's possible to win without winning all three -- Mark Warner narrowly won re-election for Senate while losing both Loudoun and Chesapeake by 0.5 percentage points -- but winning all three is a good indication of a statewide win," said Geoffrey Skelley, associate editor of Sabato's Crystal Ball at the University of Virginia. Clinton's running mate, former Virginia governor and U.S. Sen. Tim Kaine, could boost her chances in these swing counties.
To win the turnout battle in Wisconsin, Democrats will be looking to Dane County, home to the state capital of Madison and the University of Wisconsin. Clinton should win big in Dane, but it's unclear how much of her potential university vote will bleed to Johnson and Stein.
On the Republican side, it's the overwhelmingly white trio of Waukesha County, Ozaukee County and Washington County in suburban Milwaukee. All are usually solidly Republican, but Trump has been a harder sell than most GOP candidates. In Waukesha, according to the Marquette University Law School poll, Trump led Clinton during the summer by 13 points -- compared to the 30-to-40 point margins Republicans typically get, according to the Milwaukee Journal Sentinel. Whether Republicans in these counties come home and support Trump on Election Day will have a major influence on the result.
Finally, Brown County is a swing area anchored by Green Bay. Trump will hope to ring up a better-than-average showing there.