Donald Trump and Joe Biden have radically different views on many subjects.
Each could enhance, or weaken the special relationship, in a variety of areas.
Here, we look at who would be best for Britain.
Post-Brexit trade deal
There is no dispute about where Donald Trump stands on Brexit. He has been an enthusiastic supporter from the start.
A trade deal with the UK would be near the top of his in-tray in a second term.
In recent weeks both the US and UK expressed optimism as talks entered a fifth round.
US Trade Representative Robert Lighthizer said he was "very pleased" with progress and predicted a trade deal "reasonably soon".
He said: "These things take time but we are making great headway. I am optimistic across the board."
Joe Biden did not support Brexit and says it is "not how we would have preferred it to be".
He was vice president when Barack Obama appealed for the UK to stay in the European Union. Mr Obama said the UK would end up at the "back of the queue" for a trade deal.
Last month, Mr Biden wrote on Twitter: "We can’t allow the Good Friday Agreement that brought peace to Northern Ireland to become a casualty of Brexit."
Earlier this year Mr Biden told the Telegraph that Britain "still is in a special relationship, notwithstanding the way President Trump keeps talking about Europe and the world. Nato is critical."
He has committed to consulting allies and acting in a less unilateral manner than Mr Trump, with his commitment to an "America First" agenda.
Nato itself is reportedly considering an early summit in March in Brussels to welcome Mr Biden if he is elected.
Mr Trump has called Nato “obsolete” and lambasted allies as “delinquent,” forcing them to contribute more to defence budgets.
He also announced his intention earlier this year to cut the US troop contingent in Germany.
A Nato diplomat said: "Most allies want a Biden victory next month."
Another said a March summit "would give Biden a platform to bring Europe and North America back together, and also give Nato a chance to put the Trump era behind it."
Iran nuclear deal
Mr Trump announced the US withdrawal from the Iran nuclear deal in 2018.
The UK, along with France, Germany, China, and Russia, remains a signatory. The deal is on life support.
It has been one of the most divisive elements of Mr Trump's foreign policy. He is using a "maximum pressure" sanctions campaign against Iran, believing Iran will ultimately seek to negotiate a deal that is better for the West.
Mr Biden has been a vocal defender of the deal, which was negotiated while he was vice-president.
He has indicted that, if Iran returns to "strict compliance" then the US will rejoin the UK and others in the deal.
Mr Trump, as he always promised to do, withdrew the US from the Paris climate accord.
By contrast, Mr Biden has pledged to rejoin the agreement which aims to limit increases in global average temperatures to “well below 2C above pre-industrial levels". Boris Johnson's government would welcome a US return to the agreement.
Next year the UK is hosting the United Nations Climate Change Conference (COP26) in Glasgow.
It will be a major opportunity to promote Britain's efforts on climate change.
Mr Trump, if re-elected, is not expected to attend.
Boris Johnson has a warm relationship with the current US administration, helping to bolster the special relationship.
Mr Trump has called Mr Johnson a "great guy" and the prime minister also has a very good relationship with Jared Kushner, Mr Trump's highly influential son-in-law.
Lord Darroch, the former British ambassador to Washington, said recently that Mr Johnson would be Mr Trump’s “best friend in Europe” if the president is re-elected.
Mr Trump has golf courses in Scotland, visited the UK three times in four years, and has a well known admiration for Sir Winston Churchill.
If Mr Biden is elected, Mr Johnson would be starting from scratch, and the Democrat may prioritise relations with Brussels, and other European leaders.
When Mr Biden was vice president, President Barack Obama had an excellent relationship with Angela Merkel, and its is possible Mr Biden could see the German Chancellor as his greatest friend across the Atlantic.