As Boris Johnson prepares to visit Northern Ireland tomorrow on his tour of the union, a group flexing their muscles from across the Atlantic may well be weighing on his mind.

40 prominent Irish-American politicians and business leaders have written a letter to Julian Smith, the new Northern Ireland Secretary, expressing their concerns about maintaining a frictionless border on the island of Ireland as the 31 October draws closer. They are certainly neither the first nor the last group to warn of the dangers of a no-deal Brexit to peace and stability in the region, but it is a significant intervention, because, indirectly, they represent a real-life obstacle to the new Prime Minister’s hopes of a UK-USA deal.

While Liz Truss, the new International Trade Secretary, declared today that such a deal would be her “top priority” after meetings with the US Ambassador yesterday, these high-powered Irish-Americans, who form what is pithily called the Ad Hoc Committee to Protect the Good Friday Agreement, have amplified Nancy Pelosi’s warning from last April that a UK-USA trade deal would be “impossible” if Brexit endangers the Good Friday Agreement. And the challenge for Truss, Johnson and Trump is that Trump doesn’t have the final sign-off: rather, any trade deal would have to pass through Pelosi’s Democrat-held Congress.

At the time, Pelosi spoke about America’s deep investment in the Good Friday Agreement as “an ideal, a value, something we all take pride in”, and as an American legacy she would consider to be undermined by a hard border after a no-deal Brexit. This new intervention is just another reminder that the American legislature maintains a deep, emotional and longstanding alignment with Ireland’s interests, regardless of Donald Trump’s famous interest in making deals.

The possibility of a “very substantial” trade agreement with the U.S.A. is being presented by the Johnson administration as a boon amid preparations for a potential No Deal. As he travels around Northern Ireland tomorrow, he will be aware that he is walking on a very thin tightrope.