A lot depends on whether the Trump administraton can maintain that it has expedited authority to negotiate trade deals and that this authority includes any new deal with the UK. I don't know what that status is at present, but, with the Irish Question at the fore, Trump would find it hard to get such authority over the opposition of the Friends of Ireland caucus.

Any future US-UK trade deal would almost certainly be blocked by the US Congress if Brexit affects the Irish border and jeopardises peace in Northern Ireland, congressional leaders and diplomats have warned.

Boris Johnson has presented a trade deal with the US as a way of offsetting the economic costs of leaving the EU, and Donald Trump promised the two countries could strike "a very substantial trade agreement" that would increase trade "four or five times".

Trump, however, would not be able to push an agreement through a hostile Congress, where there would be strong bipartisan opposition to any UK trade deal in the event of a threat to the 1998 Good Friday agreement, and to the open border between Northern Ireland and the Republic.

"The American dimension to the Good Friday agreement is indispensable," said Richard Neal, who is co-chair of the 54-strong Friends of Ireland caucus in Congress, and also chairs the powerful House ways and means committee, with the power to hold up a trade deal indefinitely.

"We oversee all trade agreements as part of our tax jurisdiction," Neal, a Democratic congressman from Massachusetts, said in a phone interview. He pointed out that such a complex trade deal could take four or five years, even without the Northern Ireland issue.

"I would have little enthusiasm for entertaining a bilateral trade agreement with the UK, if they were to jeopardise the agreement."

Pete King, the Republican co-chair of the Friends of Ireland group, said the threat to abandon the backstop and endanger the open border was a "needless provocation", adding that his party would have no compunction about defying Trump over the issue.

"I would think anyone who has a strong belief in Northern Ireland and the Good Friday agreement the open border would certainly be willing to go against the president," King said.

At the very least this might add uncertainty to the prospect of a quick bilateral trade agreement. And, in any case, such an agreement is highly unlikely until after 2020, when any authority Trump might now have on trade would likely expire.

"It is not necessary to have hope in order to persevere."