Amid reports that Boris Johnson’s government might invalidate last year’s agreement for a “no-border” Brexit deal, US Reps. Richard Neal and William Keating joined two other colleagues in a stern message to Johnson on Sept. 15 that prompted a Tweet from Democratic presidential nominee Joe Biden the next day. 16.

“We can’t allow the Good Friday Agreement that brought peace to Northern Ireland to become a casualty of Brexit,” wrote Biden, in a Tweet that included a copy of the Neal letter to Johnson. “Any trade deal between the US and UK must be contingent upon respect for the Agreement and preventing the return of a hard border. Period.”

The letter from Neal and Keating was co-signed by Rep. Eliot Engel, a Democrat who chairs the House Foreign Relations Committee, and by Peter T. King, a Republican from New York. In it, they warn that Congress “will not support any free trade agreement between the United States and the United Kingdom if the [UK] fails to preserve the gains of the Good Friday Agreement and broader peace process.” 

They continued: “… we therefore urge you to abandon any and all legally questionable and unfair efforts to flout the Northern Ireland protocol of the [Brexit] agreement.” An Ireland “divided by a hard border risks inflaming old tensions that very much still fester today,” they wrote.

Johnson has suggested that his government might override parts of the Brexit deal as it negotiates a settlement with the EU that is supposed to be resolved by Oct. 15. British Foreign Secretary Dominic Raab visited Washington in September for meetings with Congressional leaders, including House Speaker Nancy Pelosi, who has warned that Britain won’t secure a much sought-after trade deal with the United States — which would require congressional approval — if it undermines the peace accord.

Raab insisted the U.K. has an “absolute” commitment to the Good Friday Agreement. He described Britain’s planned law as “precautionary” and “proportionate.”

Secretary of State Mike Pompeo said the US was clear on the importance of the peace accords but understood “the complexity of the situation. We trust the United Kingdom. I am confident they’ll get it right,” he said after meeting Raab.

Associated Press reports contributed to this story.