Key House and Senate races to watch to find out which party will control Congress

The 2024 election is on a knife's edge, with polls predicting one of the closest elections in modern American history.

An early wave of polls ends at 6:00 p.m. and 7:00 p.m. on the East Coast, giving viewers valuable insight into whether the national environment is trending red or blue – including which messages are top of mind in contentious areas and in what direction the suburbs are trending and whether black voters will deliver for the Democrats.

The answers will give Americans an early look at the key races for control of Congress and other voting contests, as well as insights into how Vice President Kamala Harris fares against former President Donald Trump.

Some of the first polls will close at 6:00 p.m. ET in parts of Indiana and Kentucky. The only potential competitive congressional race to watch in these states is Indiana's 1st District, where Democratic Rep. Frank Mrvan is running for re-election. Neither party's outside groups have entered the race, but if GOP outsider Randy Niemeyer pulls off an upset, it could be a harbinger of a red wave.

Both parties believe a wave is unlikely, so other districts will provide better clues as to which way the wind is blowing in the contest for Congress and the big presidential contest between Harris and Trump.

Here are some early leitmotifs to keep in mind:

Two Virginia counties offer insight

Two Virginia congressional districts where polls close at 7 p.m. ET could tell the story of the evening: the 2nd District, centered around Virginia Beach, and the 7th District, which includes Fredericksburg. Both districts can be valuable early indicators of how the elections are shaping up in the suburbs that are crucial to Democrats' victory in the down-ballot.

Republican Rep. Jen Kiggans is favored in Swingy 2nd district. If she loses to Democratic rival Missy Cotter Smasal, GOP activists will worry they're in for a bad night. It could also mean that Democrats' messages about protecting abortion rights, upholding democracy and rejecting extremism — key themes of Cotter Smasal's outsider campaign as well as Harris's — resonate.

Kiggans is one of 16 House Republicans running for reelection in districts won by President Joe Biden in 2020. Biden won this seat by just 2 points in 2020, according to an NBC News Decision Desk analysis. Therefore, this race could also provide some initial clues about the presidential race.

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Virginia's closely divided 7th District is pitting two political newcomers and Army veterans against each other: Democrat Eugene Vindman and Republican Derrick Anderson, while Rep. Abigail Spanberger, D-Va., is vacating her seat to run for governor . Democrats are slight favorites in the district, a mix of urban and rural parts of Northern Virginia, which Biden carried by 6 points four years ago. Vindman played a role in reporting on the Ukraine phone call that led to Trump's first impeachment, and his brother Alexander testified in those impeachment hearings. Anderson became persistent by using a fake family photo in which he posed with his friend's wife and three children to create the impression in campaign materials that he was a family man.

“If Jen Kiggans lost in Virginia Beach, it would be a surprise. If (Eugene) Vindman lost at Northern Virginia, Virginia-7, it would be a surprise. And if those races fail, that will reflect our expectations, which is a really close House race,” said Kyle Kondik, editor-in-chief of Sabato's Crystal Ball, a nonpartisan election forecaster at the University of Virginia.

If one party wins both races, they could be in for a strong night across the country.

North Carolina 1st District: Black Vote Test

This North Carolina district, where polls close at 7:30 p.m. ET, will provide a valuable early indicator of the Black vote, one of the most important questions of the 2024 election cycle. The closely divided 1st District is 39%, according to census data of blacks, and both parties have aggressively courted voters there.

The rural district has become a tougher climb for Democrats since Republicans redrew the state's congressional map late last year. Biden would have won this redistricted district by just one point in 2020. And national Republicans got their preferred candidate, GOP Army veteran Laurie Buckhout, through the primary to face Democratic Rep. Don Davis, a first-term Democrat.

Crucial to the district's success and a possible Harris victory is high black voter turnout and avoiding significant defections to Trump, who is trying to alienate some of the young black men. If she fails in any of these tasks, her path to victory will quickly become more difficult, as will Democrats' hopes in a number of election contests.

Are Trump's tactics working? Or can Harris hold his own with black voters? Are you highly motivated to vote? Or are the number of participants lower than usual? The results in this district could help answer these questions.

And the answers will have profound implications across the country, including in North Carolina, one of seven battleground states expected to decide the presidential election.

Ohio's 9th District: Trump Country test

Republicans looking to make gains among blue-collar voters key to the Trump coalition could get their first clues in Ohio's 9th District, where polls also close at 7:30 p.m. ET.

Democratic Rep. Marcy Kaptur, first elected in 1982, faces Republican Rep. Derek Merrin in Toledo County, which Trump won by three points in 2020. but Republicans are optimistic Merrin can defeat them this year.

Kaptur is one of five Democrats running for re-election in districts Trump won in 2020, and her race could be the first indication of whether Democrats can keep up in similar districts. And Kaptur's ability to surpass Harris will test the electorate's appetite to split their votes between the two parties, a dynamic that also has implications for many Senate races.