As the votes are totted up in states across America, the implications, domestic and international, will begin to sink in.
Below are three scenarios which could unfold in the hours ahead.
Beyond the shifts in power on Capitol Hill though, there are overarching implications if, as the polls suggest, the Republicans have a successful night.
The extent to which Donald Trump is the future, and not just the past, will be revealed over the hours ahead.
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The Make America Great Again (MAGA) movement has evolved from being a fringe of the Republican Party, it is now the dominant force.
And with his candidates, Donald Trump has essentially built a loyal framework to underpin a run for the presidency in 2024.
Crucially, because his candidates are overwhelming election deniers, insisting - as he does - that the 2020 election was stolen by Joe Biden, they set the stage for chaos in 2024.
But between now and then, here is how things could play out:
The Democrats hold the Senate but the Republicans take the House of Representatives
Domestic implications: President Biden and his party would face a flurry of House-driven investigations. Everything from the withdrawal from Afghanistan to the FBI investigation of Trump - to the southern border policy and Biden's COVID policies could face House inquiries.
Mr Biden's son, Hunter, could face an investigation for alleged tax fraud. The January 6th committee into the Capitol insurrection would be disbanded.
There is an outside chance that Republicans could attempt to impeach President Biden. Some believe that his liberal border policies constitute "high crimes and misdemeanours" which would qualify him for impeachment.
International implications: Although there has been rare bipartisanship on Ukraine up until now, there is growing unease among populists on the right about the cost of helping Ukraine. The modern Republican Party is far more insular than the party of Reagan or the Bush dynasty. So, expect an attempt to rein in spending and less support for Ukraine.
On the climate agenda, a Republican-controlled House will make a green agenda far more complicated with Republicans able to represent their pro-coal and oil constituents more effectively.
The Republicans take both the House of Representatives and the Senate
Likelihood: Moderate to high
Domestic implications: Mr Biden becomes a lame duck president. His agenda would be blocked at every turn - even issues where there was bipartisan co-operation in Biden's first two years, such as gun control and infrastructure.
A national 15-week abortion ban would be brought to a vote by the right of the Republican Party. Democrats would be able to use the filibuster mechanism to prolong debate on the issue.
International implications: The implications outlined in scenario one would apply, but would have more chance of playing out.
The key issue to watch would be continued American support for Ukraine. It would depend on the power handed to the ultra-MAGA right of the Republican Party. The signals sent about American priorities could have profound geopolitical implications.
On climate change, America can essentially be written out of any progressive green agenda - which would massively impact global climate goals.
The Democrats hold both the House of Representatives and the Senate
Domestic implications: Joe Biden's liberal agenda moves forward. The extent to which policies could be realised depends on the strength of the Democrat's Senate majority. But if the Democrats expand their majority, expect to see an attempt to deepen the president's Build Back Better agenda, which was watered down in order to pass elements of it earlier in the year.
Key policies on issues like climate change will be revived. Watch for efforts to protect key issues: abortion rights could be codified into law and national protections for same-sex marriage and voting rights are likely.
If the Democrats managed to expand their majority, they might be able to remove the filibuster: a mechanism that requires legislation to get 60 votes to pass and which has frustrated their attempts to push through legislation.
International implications: Beyond the climate change pledges, expect a renewed commitment to continue to back Ukraine both diplomatically but financially and militarily too. Essentially, business as usual for Mr Biden.
(c) Sky News 2022: Three possible scenarios as Americans head to the polls - not just for the US, but for the world