If Donald Trump wins against the odds, the UK prime minister may well interpret that as vindication that, with Brexit at least, he is on the right side of history. Trump, who won power the same year as the 2016 Brexit referendum, has been perhaps the most outspoken world leader in favor of Britain’s departure from the EU.

If the mercurial president wins a second term, Johnson is more likely to feel emboldened to plump for no deal with the EU, not least as a US-UK trade agreement may become an early priority of a re-elected Trump team.

Such an agreement would be a significant victory for both Johnson and Trump. For the prime minister, it would give some credence to his aspirations for a new “global Britain” in a context where the UK’s relationship with the world’s other superpower, China, has eroded badly since the pandemic began. A trade deal would also be a boon for Trump, who is criticized in many quarters for being an anti-globalization, protectionist president.

If, however, Joe Biden wins next week, Johnson could have more to lose than any world leader. This is not because Biden is anti-British; on the contrary, he was, for instance, a staunch supporter of London over the Falklands war, which divided the Reagan administration despite the latter’s admiration of Margaret Thatcher.

The reason for Johnson’s concern is that Biden views him as a political soulmate of Trump, despite the many differences between the prime minister and the president on key issues. This underlines the degree to which Johnson (who the White House has called the “British Trump”) may have mis-stepped diplomatically by putting so many of the UK’s diplomatic eggs in Trump’s basket.

Moreover, Biden has long been opposed to the UK’s departure from the EU; has said he would prioritize a trade deal with the EU over the UK; and has key concerns about the implications of Brexit for the Good Friday peace agreement in Northern Ireland (the former vice-president has Irish ancestry).

That is why a Biden win could significantly affect Johnson’s calculus on whether to go for a trade agreement rather than no deal with the EU. If Biden wins, the prospects of securing a UK-US trade deal would be shakier than before.

To be sure, there are key areas ripe for agreement whoever wins, including lowering or eliminating tariffs on goods. However, there are also icebergs on the horizon.

Specific areas of potential disagreement on trade include the prospect that harmonising financial regulations, with the international dominance of Wall Street and the City of London, would not necessarily be straightforward. Nor will it be easy to secure agreement in other sectors, including agriculture, where there are divergences of views and strong interest groups,.

If Biden wins, perhaps the best Johnson can hope for is that the new president will quickly put aside personal and partisan differences and forge a constructive partnership built on the traditional ties between the two nations founded on demographics, religion, culture, law, politics and economics.

This will be supplemented by longstanding security cooperation, which has long been at the core of the relationship given the close partnership between the two nations in areas such intelligence. On this specific agenda, it is even likely that Johnson will have more in common with Biden than Trump, given the latter’s views on issues such as the future of NATO and the West’s relationship with Russia.

There are many uncertainties ahead in the special relationship, whether Trump or Biden wins. Johnson will be more nervous in the short term, however, if there is a change of president, and is likely to seek to play the role of a trusted but candid friend to Biden in a bid to make the relationship work as smoothly as possible. This may provide some protection for relations in what could be a rocky initial period, especially if strong personal chemistry fails to take root.

However, even this safety-first strategy is not without risk. While seeking the upside in the new relationship, Johnson would be wise not to overestimate the UK’s ability to shape US power, or be blind to the prospect that Biden’s outlook may care less for core UK interests than in the past, as he increasingly looks to Berlin and Paris for post-Brexit European leadership.