What to watch for on Election Day as Harris and Trump face off

• In hours, polls will close in the 2024 presidential election.
• Kamala Harris and Donald Trump have fought over Pennsylvania, but both candidates have multiple paths.
• Here's what we're watching for.

Election Day 2024 has arrived. Billions have been spent pushing voters to back specific candidates and causes.

Vice President Kamala Harris upended the presidential race with her late entry. Pre-election polling showed her race against former President Donald Trump could be the closest in history. The margins are so tight that a potential polling error could result in an unexpected blowout.

More than 78 million Americans have already voted early, either in person or by mail. Final turnout may dip slightly from the historic mark set in 2020, but a pre-election Gallup poll showed voter enthusiasm above 2008 levels.

It's not just control of the White House. Congress is up for grabs, too. Republicans are favored to retake the Senate, thanks to key races occurring on favorable turf. The House is tougher to project. Republicans could net a few seats, or Democrats could flip the chamber with an equally narrow majority. Just like in 2018, New York and California, hardly competitive in the presidential race, loom large.

Here's what we're watching.

How Trump or Harris will win the White House

Harris' easiest path to victory remains what was once President Biden's last resort: holding Pennsylvania, Michigan, Wisconsin, and Nebraska's 2nd Congressional District — the so-called "Blue Wall" and the "Blue Dot." Barring any unexpected upsets, Harris would then win the Electoral College 270 to 268.

Trump's easiest route to victory also goes through Pennsylvania, the most important swing state. Winning Pennsylvania would likely allow Trump to retake the White House by holding North Carolina and flipping back Georgia. In that scenario, he could even sustain the blow of losing an Electoral College vote in Nebraska, which pre-election polling shows is very likely.

While so much focus is deservedly on Pennsylvania, Trump spent significant time in the race's final days in North Carolina, even though only two Democrats, Presidents Obama and Carter, have won the Tar Heel state since 1976.

These are the bellwether counties to watch.

Bucks County, Pennsylvania: Pennsylvania Gov. Josh Shapiro recently called this Philadelphia metro-area county, "the swingiest of all swing counties in the swingiest of all swing states." Hillary Clinton won Bucks by less than 3,000 votes in 2016 and lost both the state and the White House. President Biden won it by just over 17,000 votes four years ago en route to taking his native turf by a roughly 80,000 vote margin. If Trump wants to retake Pennsylvania, doing well here is key.

Waukesha County, Wisconsin: Once traditional GOP ground, this Milwaukee-suburban county area has moved closer to Democrats-like suburbs nationwide in the Trump era. Trump routed Clinton here in 2016, but his margin decreased by just over 7 points four years later, as he lost the state by a roughly 21,000-vote margin to Biden.

Maricopa County, Arizona: In 2020, the home to Phoenix, the fastest-growing county in the nation, swung to Democrats by just over 2 points. President Biden went on to win the state by just roughly 10,000 votes. In contrast, then-Gov. Doug Ducey, a popular Republican, won the county by almost 14 points two yeas prior.

Here's what early exit polls could tell us:

Exit polls will provide the first major at who turned out to vote early in the evening after the embargo lifts at 5 pm Eastern Time, though specific data on who may win a state is held back until polls close in that area.

Though exit polls can offer valuable data, especially about demographics, they are often misleading and are not predictive of an eventual winner.

Mark Blumenthal, a contractor at YouGov and previous head of election polling at SurveyMonkey, told Business Insider he will interested in how the vote ends up breaking down by gender and race, particularly given the speculation that Democrats are losing ground among Black and Latino voters.

Overall, though, he cautions against making too much of exit polls on election night itself. "As a means of trying to see who is going to win, between the time when they start being released, which is usually 5 pm Eastern tomorrow, and when we get a final count, my advice is to ignore them," Blumenthal said. "Utterly, completely ignore them. Go for a walk. Go take a yoga class, watch something on Netflix, because they're not designed to tell us who's going to be the winner of the presidential election."

Republicans are favored to retake control of the Senate.

Michigan: Rep. Elissa Slotkin is trying to hang onto this open senate seat that her fellow Democrats have held for 20 years. Former Congressman Mike Rogers, once a Trump critic, hopes the former president will push him over the top. Rogers and his allies have zeroed in on Slotkin's support for the White House's fuel efficiency standards that do not mandate electric vehicle production but would likely result in the auto industry producing more EVs. Pre-election polling: Slotin was up by roughly 3 points, per RealClearPolitics' average

Ohio: Sen. Sherrod Brown, a Democrat, has managed to survive past challenges in the former swing state that now tilts toward Republicans. His challenger, Cleveland Businessman Bernie Moreno, has hammered Brown over immigration. According to AdImpact, the Ohio Senate race is the most expensive non-presidential race ever. Pre-election polling: Moreno was up by less than 1 point, per RealClearPolitics' average.

Montana: Sen. Jon Tester, a Democrat, is facing an even tougher challenge in Montana, a state Trump carried by over 16 points in 2020. Republicans worked hard to elevate Tim Sheehy, a wealthy aerospace executive, even though expectations of a bruising primary did not come to pass. Like other Republican hopefuls, Sheehy has done everything possible to tie Harris to Tester. Pre-election polling: Sheehy was up by 6.5 points, per RealClearPolitics' average.

The House is much closer.

Arizona 1st District: Rep. David Schweikert, a Republican, has represented Scottsdale and its surrounding areas in Congress for almost 14 years. Gov. Katie Hobbs and Sen. Mark Kelly easily won the area, one of the most educated districts still held by a Republican. His challenger, former state Rep. Amish Shah, has highlighted his centrist record and is hoping Trump's unpopularity will sink Schweikert.

New York 19th: Rep. Marc Molinaro was one of four Republicans who flipped a Democratic-held seat during the 2018 midterms in New York, a margin that proved vital to the GOP retaking the chamber. Democrats are hoping Josh Riley, a lawyer and policy analyst, can win in a rematch two years later with the help of Harris at the top of the ticket. More than $35 million has been spent on the race, making it the most expensive House contest in the nation.

California 22nd: Rep. David Valadao is one of 18 House Republicans representing a district President Biden won in 2020. Valadao is also one of the only two remaining House Republicans who voted to impeach Trump after the January 6 Capitol riot. Former state Rep. Rudy Salas is like many Democrats in key races, hoping to win in a rematch after falling short two years ago.