More than half (52 percent) of Americans back a trade deal with the UK, according to polling data released this month.
But support for a deal fell by six points, to 46 percent, if concerns over Northern Ireland were raised.
Concern about the Northern Ireland Protocol has been rife in Washington, with US President Joe Biden repeatedly stressing his Irish heritage and once famously rejecting an interview with a British broadcaster.
House Speaker Nancy Pelosi has also specifically threatened to block a trade deal if the Protocol is changed.
US President Joe Biden has repeatedly stressed his Irish heritage
TOM BRENNER
Nancy Pelosi threatened to block a trade deal with the UK if the Protocol is changed
Oliver Contreras
And Patrick Basham, Director of the Democracy Institute, said the issue is a "real challenge and hurdle to overcome” in the US.
He said: “The default position for Americans over a trade deal with the UK is to be mildly, moderately positive and optimistic.
“That’s partly because a lot of Americans are still largely in favour of free trade deals.
“Also there is the affinity and reservoir of goodwill towards the UK, its principal ally.
“If you are going to pluck a country out and suggest that America might have a trade deal, you do worse in every case other than the UK.”
But Mr Basham noted that, despite more being in favour of a deal, Americans are now "more cynical" about the prospects.
He told Express.co.uk: “The fact that the figures for such a trade deal are not off the chart reflects the fact that Americans are more cynical about trade deals than they were.
“There has been a flip on the American side. It wasn’t that long ago that the opposition to free trade was very much on the political left, the protectionist pro tariff labour unions and politicians who represented them.
“Free market and big business types on the right were in favour.”
The Democracy Institute/Express.co.uk poll featured 1,500 likely voters across the USA on August 2 to 4.