Futures for the tech-heavy Nasdaq 100 index were down 2% as of 3am UK time on Wednesday morning, before cutting losses to trade down 0.5%
Meanwhile, S&P 500 futures fell 1.1%, before recovering to be down 0.35% - with traders reacting to the likelihood of a "blue wave" sweeping Washington.
A decisive win for the Democrats could exacerbate volatility in asset prices, especially in the artificial intelligence trade.
Shares in tech giants such as Microsoft, Amazon and Nvidia were all trading lower in extended trading.
The outcome may also have broad implications across the economy - affecting everything from defence spending to the regulation of financial assets.
US midterms - live updates as results come in
Donald Trump's Republican Party has controlled both chambers of Congress during the first two years of his second term.
The Democrats have now taken both the House of Representatives and the Senate.
A Democratic-led Congress could exert greater scrutiny over artificial intelligence companies as public opposition to data centres grows.
Horizon Investment Services chief executive Chuck Carlson said: "The AI trade has been so important to the market, and this election will have some direct impact on that."
Concurrent Asset Management's chief investment officer Leah Bennett added: "The market believes most likely the Democrats are going to take Congress."
A so-called "blue wave" would also likely rule out any more business-friendly policies such as tax breaks.
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The US elections have weighed heavily on the markets for weeks - as well as concerns that heavy spending on artificial intelligence is yet to provide sufficient returns.
Rising interest rates and sticky inflation, which has been above the Federal Reserve's 2% target for several years, are also playing a factor.
However, in all 20 of the midterm years seen since 1946 - the S&P 500 has been higher 12 months after election day, with an average gain of 14.4%.
Despite the pullback in the early hours of Wednesday, the Nasdaq 100 remains up 12% in the year to date, with the S&P 500 advancing 10%.
Looking ahead, Seth Hickle of Mindset Wealth Management said: "I expect volatility to remain elevated over the next few weeks, regardless of the election outcome."
Freedom Capital Markets' chief market strategist Jay Woods told Sky's US partner CNBC: "Now all eyes turn to the president and the reaction that we may get from him.
"Will there be contested elections, will there be more uncertainty thrown into the ranks? That's where the market could really get riled up."
The Public Accounts Committee (PAC) has said a Notre-Dame-style fire or the King being plunged into darkness during the State Opening of Parliament were among the "real and rising risks" of continued postponements to the restoration and renewal programme.
Without action, the best case scenario would see the historic building crumble away into managed decline, the committee added.
In its latest report, published on Wednesday, the PAC estimated that the weekly cost of maintenance on the parliamentary estate is expected to grow from £1.5m to £2m in the coming years, warning that each year the programme is delayed will add £420m to the cost of delivering the scheme.
The report comes ahead of a series of votes in parliament to decide the future direction of the restoration programme, which were expected to take place earlier this year but have been delayed.
MPs first voted to leave parliament to allow the building to undergo extensive repairs in 2018, but there has been continued disagreement about whether this should proceed.
Sir Geoffrey Clifton-Brown, chair of the PAC, said that "each kick of the can down the road costs hundreds of millions in public money", and urged those looking to delay a decision further to "imagine the blow suffered by the UK's morale and reputation of some kind of catastrophic failure".
"A lack of action risks, for instance, the chamber being plunged into darkness during the State Opening of Parliament," Sir Geoffrey added.
"Consider the impact of global rolling news footage of Notre-Dame-like flames licking out of the building's roof, resulting in the destruction of our heritage, or worse, injuries or loss of life.
"These are not notional possibilities, but real and rising risks which our committee identified three years ago."
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Several options under consideration
Parliamentarians will be expected to narrow down the number of options for how the refurbishment programme will be carried out from four to two.
The first recommended option, a "full decant", could cost up to £15.6bn and last up to 24 years, while the second proposal, a partial decant, could last more than six decades and cost close to £40bn.
MPs and peers have also been asked to agree on the initial phase of the programme - capped at £3bn - which would focus on moving both Houses to temporary locations, as well as on relocating offices outside the palace.
The PAC, however, has warned that the temporary sites for MPs and peers may not be ready for when they would be required to leave the Palace of Westminster for restoration work to begin - which according to current timescales would be in 2032.
Sir Geoffrey also acknowledged that all options for the programme would "involve extremely high figures with very big ranges".
A parliament spokesperson said the PAC report "provides welcome scrutiny", adding: "Work is already happening across the parliamentary estate to ensure the ongoing safety of those working here and visiting."
Andrew is not accused of wrongdoing.
The fraud trial is against a man who has called Andrew as a witness, as reported last month.
It is understood the former prince will be compelled to give evidence before the High Court around 15 December, either in person or by video.
It is believed that Andrew is the first royal since the 19th century to be sent such a witness summons.
Sky News has approached Andrew for comment.
The case will hear allegations made by Nebahat Isbilen, a Turkish millionaire and wife of a former politician, who says her former adviser, Selman Turk, misled her and used her wealth for his own interests.
Mrs Isbilen claims she was misled by Mr Turk over a payment of £750,000 made to the former Duke of York in 2019, which she believed was for help with a passport, according to court documents.
Mr Turk strongly denies the allegations, claiming the payment was made on Mrs Isbilen's own initiative to create goodwill with Andrew and had nothing to do with a passport.
Court documents also show that there will be questions about another payment made from Mrs Isbilen's account - about £768,743 in January 2020 - which Mr Turk says was a loan to Andrew for the purchase of a chalet in Switzerland.
Mrs Isbilen has contested that claim.
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Representatives for Mr Turk said: "The judge concluded that the proposed evidence that Mr Turk wishes to adduce from Mr Mountbatten-Windsor appears relevant to allegations of dishonesty advanced against Mr Turk and that, in the circumstances, it would be unfair and unjust to prevent him from seeking to adduce that evidence at trial."
They added that Mr Turk "looks forward to vindicating his position at the forthcoming trial".
Andrew's court appearance will be the first public questioning he has faced since his interview on BBC Newsnight in 2019.
The trial at the High Court is due to start later this month.
The new qualification - which will be available to all pupils - is primarily aimed at those displaced by the full-scale Russian invasion in 2022.
It follows reports that the number of people taking Russian GCSEs had spiked in recent years, potentially because Ukrainian pupils were encouraged to take it, in the absence of a Ukrainian qualification.
Ukrainian officials were reportedly "deeply concerned" in 2025 at potentially retraumatising displaced teenagers by pushing them to learn Russian.
Education Secretary Lucy Powell said Ukrainians and their children should have "every opportunity" to connect with their heritage.
She added the course will give pupils "the chance to develop their language skills and celebrate Ukraine's rich culture and history, while also opening pathways to future study and employment".
The qualification, designed by awarding body AQA, will assess listening, speaking, reading and writing skills.
It is expected to be available from September 2028, with the first assessments due to take place in 2029.
The government said it displays the UK's "enduring support for the Ukrainian community".
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Ukrainian minister of education and science, Andrii Butenko, said: "Today marks an important milestone towards the introduction of a Ukrainian GCSE, long called for by the Ukrainian community in the UK.
"British schools opened their doors to thousands of Ukrainian children following the start of Russia's full-scale war of aggression against Ukraine, and we will always be grateful for that.
"We welcome the UK government's next step towards giving Ukrainian children a genuine choice in their education while respecting their identity."
Last week, the government announced an extra £10bn of economic and military support for Ukraine.
Two flights to Warsaw, two to Sofia, two to Rome and one to Murcia airport in Spain were subjected to the threats on Tuesday, and another flight to Sofia had been threatened a day earlier.
Warsaw-Modlin Airport temporarily shut after a Tuesday morning flight from Heathrow, operated by Polish airline LOT, was diverted there. It had been due to land at Warsaw-Chopin Airport.
At 9.54am, a bomb threat alert was raised on the Boeing 737 MAX aircraft, which was carrying 181 people.
The flight landed safely seven minutes later, Warsaw Modlin Airport, which has since reopened, said on its website, and the bomb threat was "cancelled" at 12.21pm.
No suspicious items was found following a security inspection, a spokesperson for LOT Polish Airlines said, adding that passengers were taken to Warsaw Chopin Airport by coach.
A bomb threat was also reported on a Scandinavian Airlines flight from Copenhagen to Warsaw, which landed at Warsaw-Chopin Airport, with the airport later confirming that no device was found.
In another incident, a Ryanair plane from Manchester made an emergency landing at Murcia Airport in southeastern Spain, due to a threat, with 114 passengers and seven crew members leaving the plane.
Francisco Lucas, the Spanish government's delegate in the region, said on X that the Civil Guard ruled out the presence of explosives on the plane after an inspection.
A UK government spokesperson said of the two planes that flew from Britain: "The UK has one of the most robust aviation security systems in the world, with well-tested processes for dealing with any potential threats, as well as malicious hoax threats, like those we've seen today.
"Our world-leading police and intelligence agencies work closely every day to keep us safe, using the tools they have to assess and respond to potential threats."
Bulgarian authorities found no signs of explosives on two aircraft at the Sofia International Airport that had both received bomb threats on Tuesday while en route, Bulgaria's Minister of Transportation Georgi Peev said on Tuesday, according to state news agency BTA.
The two planes, originating in Italy's Bergamo and Dublin, were inspected by officers of the Specialised Counter-Terrorism Unit after landing in Sofia.
The Bergamo flight was operated by Wizz Air and was carrying 176 passengers, while the Dublin flight was operated by Ryanair and had 185 passengers on board.
A Ryanair flight from Lisbon landed at Rome's Fiumicino airport on Tuesday morning after an emailed threat of a bomb on board was received, a Fiumicino official with knowledge of the events, said.
An inspection found no explosives on the plane.
There was also a bomb threat reported on another Ryanair flight to Rome from Dublin, with no explosives found, an official said.
It is not known who was behind the threats, but Matthew Borie of Osprey Flight Solutions, which advises airlines on flight threats, said the focus on Europe means "there could be a Russian angle".
"We need to keep in mind, though, that Iranian hybrid warfare is just as active in certain pockets of Europe as well."
Travellers shouldn't be alarmed, he said, pointing to "coordinated response mechanisms" by national authorities - including, at times, the deployment of NATO fighter jets to escort such planes.
The threats come a day after a Wizz Air flight from London to Sofia was diverted to Budapest airport after receiving a bomb threat.