Seven key swing counties, known as "bellwether counties," with some exceptions, have consistently sided with the winning presidential candidate for decades.
They are Door County, Wisconsin; Kent County, Michigan; Vigo County, Indiana; Clallam County in Washington; and Bucks, Erie and Northampton counties in Pennsylvania.
In some, Trump won or improved his performance from 2020. Other counties went for Harris, who is projected to lose the 2024 race.
Here's how these battleground counties voted in Tuesday's election.
With 98% reporting as of Wednesday morning, Trump led in Bucks County, a primarily suburban county just north of Philadelphia, by a mere 512 votes. If Trump's lead of 49.58% to Harris' 49.45% holds, it would be the first time a Republican won the county since 1988. Biden beat Trump by four points in 2020.
Trump is projected to win Pennsylvania, according to the Fox News Decision Desk.
In Erie County, Trump is leading with 50.26% of the vote, compared to the 48.82% for Harris. Erie is situated in far northwestern Pennsylvania on Lake Erie, is primarily rural and significantly smaller in terms of population.
That is a flip from 2020, when Biden won the county by 1 percentage point.
Trump is also leading with 50.63% of the vote in Northampton County, where Harris received 48.40%. Northampton in eastern Pennsylvania is suburban and home to Lehigh University, a private research college.
Biden won Northampton by less than a point in 2020.
Berwood Yost, the director of the national survey group the Center for Opinion Research, said all three Pennsylvania counties mirror many of the key demographics, such as racial composition, educational attainment and population density, that make Pennsylvania so competitive.
"Both campaigns see the path to the White House running through Pennsylvania," Yost said, adding the path to victory in Pennsylvania runs through Bucks, Erie and Northampton counties.
Biden won Pennsylvania overall by a narrow 1.17 percent margin in 2020.
Door County, Wisconsin, broke with its usual bellwether status on Tuesday night, as 50.56% of voters chose Harris, while 48.33% chose Trump, according to unofficial results Wednesday morning.
That contrasted with the state as a whole. Trump carried battleground Wisconsin, Fox News projects, with 49.75% of the vote, compared to 48.8% who selected Harris.
In 2020, Biden received 50% of the vote in Door County, while Trump received 48.6% there. Biden carried Wisconsin as a whole 49.6% to Trump's 48.9%, and the Democrat won the 2020 general election.
In Kent County, Michigan, 51.53% of voters chose Harris, while 46.69% chose Trump. As of Wednesday morning, Trump is leading in Michigan with 50.09% of the vote to Harris' 48.09%, but the Fox News Decision Desk has not yet called the race.
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A traditionally conservative county that has moved into toss-up territory in recent years, Kent's chosen candidate has also won Michigan and the general election in the last three presidential contests.
In the 2020 election, Biden won 52.1% of the vote in Kenty County, while Trump won 45.9%. That year, Biden carried Michigan with 50.6% of the vote, while Trump received 47.9%.
According to WOOD-TV, that represented the first time more than half of the vote in Kenty County went to the Democratic candidate since Lyndon Johnson's re-election victory in 1964.
"Traditionally, Kent County has been a Republican-leaning county, and its sort of dominant political culture has been in the spirit of Gerald Ford through Reagan-Bush conservatism, but that started to change right around the [Barack] Obama years," Whitt Kilburn, a political science professor at Grand Valley State University, told WOOD-TV, crediting a population boom and suburban development outside Grand Rapids for the change.
A Democratic presidential candidate carried Kent County for the first time in more than 40 years in 2008 when the majority of voters chose Barack Obama.
Despite his victory in the general election and winning Michigan's electoral votes, Obama lost Kent County by 8 percentage points to Mitt Romney in the 2012 election, according to WOOD-TV.
Kilburn credited Trump for Kent County remaining purple territory.
"Trump, for the most part, has been pretty clear. He rejects Reagan-Bush conservatism, and you can extend that back to Ford as well… pillars of moral traditionalism, commitment to free trade, commitment to strong military alliances," Kilburn told WOOD-TV. "MAGA conservatism is presented to voters as an alternative and an alternative that is radically different from those ideas of conservatism."
Trump won Vigo County, Indiana, with 58.08% of the vote, while Harris received 39.96%. The Republican presidential nominee also won statewide, receiving 59.08% of the vote, compared to 39.20% for Harris.
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Matthew Bergbower, a political science professor at Indiana State University, described a bellwether county as a "microcosm of the nation" in terms of political preferences.
Though his county, Vigo County in Indiana, deviated by voting for Donald Trump in 2020, it has chosen the winning candidate in every election since 1952.
In Clallam County, Washington, Harris received 54.29% of the vote, while Trump received 43.90%. The state of Washington also handed the Democratic presidential nominee 58.62% of the vote, while Trump received 39.10%.
Before the 2024 contest, Clallam County stood out as the only county to have voted for the winning presidential candidate in every election since 1980.
In 2020, Biden carried Clallam County with 50.4% of the vote, while Trump received 47.0%. Biden carried Washington state in 2020 with 58.4% of the vote, while Trump received 39.0%.
Before Tuesday, the people of Clallam County considered themselves proud of their history as the "last bellwether county" in America.
Pam Blakeman, chair of the Clallam County Republicans, previously told Fox News Digital that she thought the election "will be close in our County, but I see it swinging towards Trump," pointing to good Republican turnout and a ground game that she said "is the most active I have ever seen." Yet, Ben Anderstone, a progressive Washington-based political consultant, had told Fox News Digital that "a Trump win in Clallam County would be a bit of a surprise at this point."
In the 2024 election, the largest swing state of Pennsylvania handed Trump a victory with 50.78% of the vote, while Harris received 48.26%. And Trump also carried the state's three bellwether counties.
Fox News' Peter Pinedo contributed to this report.