image captionJohn Hickenlooper has taken the Colorado Senate seat from Cory Gardner
As well as the White House, Democrats and Republicans are in a crucial race for control of Congress.
In the Senate, Democratic former governor John Hickenlooper has taken Colorado’s seat from the Republican incumbent Cory Gardner.
Republican Senate Majority Leader Mitch McConnell and Trump ally Lindsey Graham have however both been re-elected.
In Alabama, Republican candidate Tommy Tuberville has taken the seat from Democratic incumbent Doug Jones.
Senators serve six-year terms, and every two years a third of the seats are up for re-election.
This year, 35 out of 100 senate seats are being contested. To gain control, a party needs 51.
The congressional election is running alongside the battle for the White House between Donald Trump and Joe Biden.
A Democrat-controlled House and Senate would have the power to obstruct the plans of a second-term President Trump, or push through the agenda of a first-term President Biden.
In the Senate, Republicans already have a 53-47 majority – meaning Democrats need an overall gain of four seats in order to win an outright majority.
Of the 35 seats up for grabs, 23 are currently Republican-held and 12 are Democrat.
Among the candidates for Senate this year were a pastor, a football coach and a former astronaut.
Meanwhile the Democrats already have control of the House of Representatives, and are looking to keep hold of it while also gaining a majority in the Senate.
Voters are also choosing representatives in their local state senates – and in Delaware, Democratic candidate, Sarah McBride has made history as the country’s first ever trans state senator.
Mr Hickenlooper was governor of Colorado for two terms from 2011 until last year, and last year stood for the Democratic nomination for president.
At the same time the state’s incumbent, Republican Cory Gardner, was considered particularly vulnerable because of his allegiance to President Trump – the state has voted Democrat in the last three presidential elections.
The Democrats have focused their campaigns on controlling the spread of Covid-19 in the US – which has had the world’s highest death toll and number of infections for much of the pandemic. Republicans, meanwhile, have campaigned on economic issues.
One Republican candidate who has won a seat in congress has previously promoted the far-right QAnon conspiracy theory. Marjorie Taylor Greene, from Republican, was also hailed as a “future Republican star” by President Trump earlier this year.