Millions of Americans are participating in midterm elections that could change the balance of power in Washington. Their outcome will determine which party - US President Donald Trump's Republicans or the opposition Democrats - controls Congress.
Once the polls close, forecast results may not be available for hours, days or even weeks. Here's what you need to know about this election.
When will the results be known and it will become clear who will control Congress?
Congress consists of two houses - the lower House of Representatives and the upper house, the Senate.
To gain a majority in the House of Representatives, the party needs 218 seats. In the Senate, a party needs 51 seats for an absolute majority. However, if the upper house is tied 50-50, the vice president has the casting vote, giving his party control of the Senate.
The first results will begin to arrive after 18:00 Eastern Time (23:00 GMT). They won't provide a clear picture of the overall outcome because these elections are in states considered "safe" for Republicans (Kentucky and Indiana) or Democrats (Vermont).
Throughout the night, BBC analysts will be focusing on the tightest battles, which will provide insight into how well things are shaping up for Trump.
There are several tight Senate races in the Eastern Time Zone, including Ohio, where polls close at 7:30 p.m. local time (00:30 GMT), and Maine, where they close at 8:00 p.m. (01:00 GMT). These elections may provide the first indications of voter sentiment.
US media may begin to announce results in some key states. They use models to predict the outcome of a state's vote before a full vote count is completed.
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This occurs when they believe that the party or candidate has secured an insurmountable advantage. However, in some states where the parties have approximately equal chances, the forecasts of journalists and analysts may take time.
The overall timing of the announcement of results will depend on how closely contested the individual states are and which states prove decisive.
What was it like in the last elections in 2022?
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In 2022, on November 9, the day after the midterm elections, Republicans and Democrats were still locked in a tight battle for control of the US Congress.
Although Republicans were considered the favorites to win the House of Representatives, the Senate seats were closely contested.
It took several days to get the full picture. On November 17, CBS News projected that Republicans had secured the 218 seats needed for a majority in the House of Representatives.
The counting of votes in the critical race for a Senate seat in Nevada took four days. Democrats' narrow victory in the state gave them 50 seats, allowing them to retain control of the Senate because then-Vice President Kamala Harris had the tie-breaking vote.
The Georgia Senate race had to be decided in a runoff after no candidate received 50% of the vote in November.
The results were not known until December 7, when Democrats won the seat.
What are the key races to watch in 2026?
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The independent analytical resource Cook Political Report finds that in seven states it is impossible to predict who will be elected to the Senate today: Alaska, Kansas, Iowa, Maine, Michigan, Ohio and Texas.
Of these, six are currently represented in the Senate by Republicans and one by Democrats. Democrats need to win four of those seats, as well as four states where Democrats are seen as likely to win, to gain 51 seats and control of the Senate.
If Democrats fail to hold onto the seat in Michigan, winning a Senate majority will become much more difficult. Abdul El-Sayed, a left-wing political outsider, is running against Republican Mike Rogers, a Trump ally and former FBI agent.
But polls suggest they have a chance even in Texas, where former teacher James Talarico could become the first Democrat to win a Senate seat from that state since 1988 if he defeats Ken Paxton, the state's current attorney general and a close Trump ally.
The current Republican majority in the House of Representatives, the lower house of Congress, is slim. They have 218 representatives - just enough to control the chamber - plus one independent MP who votes with them.
Some of the most important House seats are elected in East Coast states, according to the Cook Political Report. These include New York's 17th District, Pennsylvania's 8th District, and Virginia's 1st District.
When will polling stations close?
Voter poll results will be released at 5:00 pm ET (22:00 GMT), giving us an idea of what voters are thinking about first.
Then, at 6:00 pm ET (23:00 GMT), the first polls will close in Kentucky and Indiana. After that, all eyes will be on seven key states where the battle will be fierce:
How are votes counted?
Each state administers its own elections and has its own rules for counting and processing votes.
The order in which votes are counted varies from state to state. Some states, such as Florida, process early and mail-in ballots until Election Day, while others don't begin counting until Nov. 3.
Local election boards—sometimes appointed, sometimes elected—verify, process and count individual votes.
Most states require an official post-election audit, which involves manually counting ballots against machine counts, although rules vary. A full manual count is typically only done during a recount.
Each state and locality has strict rules governing who can participate in the vote count, the order in which votes are processed and at what points the process is open to the public, including how political party observers can monitor the count and, in some states, file formal protests.
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What could cause delays?
In the case of a tight race, the process will take longer, which could lead to a recount and the results could be challenged in court.
Counting mail-in ballots takes significantly longer because staff must process envelopes and verify signatures.
More than 30 states allow ballots to be processed before Election Day, but others do not, meaning results can take days to arrive.
For example, in California, all ballots must be mailed by or on Election Day and will be counted even if they arrive within seven days of the election.
Since each state has different rules and regulations, some delays may be built into the system itself.
Alaska's ranked-choice voting system could mean votes take significantly longer to count than in other parts of the country, and disputes over ballots and possible lawsuits could complicate matters.
What happens if the results are disputed?
After every vote is counted in the final results and procedures such as recounts are completed, the election results are certified—first at the local and county levels. The results are then certified statewide, typically by the Secretary of State's Board of Elections.
In most cases, the governor then signs certificates of the winners of the House and Senate elections, which are sent to Congress.
The results may be disputed. If there is a small margin, candidates can request a recount, and some states trigger a recount automatically if the difference in votes is within a certain threshold.
In addition, candidates can file election-related lawsuits alleging irregularities.
Congress has the final say. Losing candidates in both the House and Senate elections can file lawsuits, with slightly different procedures in each chamber.
Both chambers could theoretically refuse to grant a mandate or order an investigation.
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When do the winners take office?
Retiring members of Congress leave office on Sunday, January 3. Because this date falls on a weekend, new members may take office on the same day unless Congress passes a law setting a different date.
With contributions from James Fitzgerald, Yazmina Garcia-Fernandez, Tom Geoghegan and Mike Hills.
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