LONDON — If there was any residual doubt about how Prime Minister Boris Johnson’s latest Brexit maneuvers are playing in Democratic political circles in Washington, it was erased on Wednesday evening by a carefully timed, unmistakably pointed tweet from the Democratic presidential nominee, Joseph R. Biden Jr.
“We can’t allow the Good Friday Agreement that brought peace to Northern Ireland to become a casualty of Brexit,” Mr. Biden wrote. “Any trade deal between the U.S. and U.K. must be contingent upon respect for the Agreement and preventing the return of a hard border. Period.”
Mr. Biden’s blunt words came shortly after a meeting between the British foreign secretary, Dominic Raab, and two of Ireland’s most influential advocates in Congress, House Speaker Nancy Pelosi and Representative Richard E. Neal, a Massachusetts Democrat who is chairman of the Ways and Means Committee.
The lawmakers warned Mr. Raab that Congress would block a trans-Atlantic trade deal if the bill Mr. Johnson is pushing — which would rewrite parts of Britain’s withdrawal agreement with the European Union that relate to Northern Ireland — leads to the resurrection of a hard border on the island of Ireland.
For Mr. Raab, whose visit to Washington also included meetings with the Trump administration, it all added up to a stark reminder that in four months, Britain could face a new president with very different views than President Trump’s on Brexit, trade and the “special relationship” between the United States and Britain.
“The British government — so focused on their own domestic politics and battles with the E.U. — seems to have underestimated the backlash their approach would have in Washington, and particularly with Democrats committed to the Good Friday accord,” said Philip H. Gordon, a former assistant secretary of state for European affairs who is advising the Biden campaign.
Ireland’s champions on Capitol Hill have long pressed Britain to avoid any actions in its exit from the European Union that would threaten the Good Friday Agreement, which ended decades of sectarian violence in Northern Ireland. The agreement was brokered by former President Bill Clinton and remains a prized legacy for Democrats.
With Mr. Trump in the White House, however, Mr. Johnson has had a like-minded ally whose enthusiasm for Brexit and instinct to undermine the European Union is at least as strong as his commitment to protect Ireland.
Mr. Biden, by contrast, opposed Brexit and served a president, Barack Obama, who once warned Britons that if they left the European Union, they would put themselves at the “back of the queue” in trade talks with the United States.
Mr. Biden is also a proud Irish-American with close links to other members of Congress who support Ireland, a group commonly referred to as the Irish lobby.
“The pursuit of peace in Northern Ireland has been one of the vice president’s priorities since his days on the Senate Foreign Relations Committee,” said Shailagh Murray, a former aide to Mr. Biden.
“The Irish cause is in his veins,” she added, noting that one of his great-grandfathers, Edward Francis Blewitt, wrote poems that were tinged with Irish republican sentiment. Mr. Biden liked to quote from them, though he more commonly cites Seamus Heaney and William Butler Yeats.
In London, the government played down a rift with the United States over Northern Ireland. A Downing Street spokesman told reporters that nothing in the legislation would restore a hard border between the North and the Irish Republic.
Mr. Raab posted a photo of himself and Ms. Pelosi, a California Democrat, on Twitter, standing in front of three American flags, and said he welcomed a discussion about “Brexit and UK’s stalwart commitment to Belfast (Good Friday) Agreement.”
During their meeting, Mr. Neal said, Mr. Raab told him and Ms. Pelosi that the European Union, not Britain, was threatening the agreement. And he promised flatly that there would never be a border bisecting the island of Ireland.
“I’m not persuaded,” Mr. Neal said in a telephone interview. “I’m receptive to what he said — he’s the foreign minister. But I also said to him, ‘You’re reassuring me about something that you’ve already reassured me about.’”
Mr. Neal said Mr. Biden’s tweet, as well as a letter to Mr. Johnson from him and three other prominent Democratic lawmakers, had been coordinated with the Biden campaign to send a strong message. Some British officials questioned why the Johnson government did not seem prepared for the backlash.
“This story has been out there for a long time,” said Kim Darroch, a former British ambassador to the United States. “But the way Boris spoke about it in Parliament suggests to me that he was surprised by the reaction.”
Pro-Brexit lawmakers dismissed Mr. Biden’s comments, saying he was simply appealing for Irish-American votes. A former leader of the Conservative Party, Iain Duncan-Smith, lashed out at the former vice president, telling The Times of London that he should stop lecturing Britain about Northern Ireland.
“If I were him,” Mr. Duncan-Smith said, “I would worry more about the need for a peace deal in the U.S.A. to stop the killing and rioting before lecturing other sovereign nations.”
Such criticism recalls the angry response of Brexiteers to Mr. Obama’s comment about Britain being last in line for a trade deal — something that aides to Mr. Obama said he did with the encouragement of Prime Minister David Cameron, who was trying to stave off defeat in the Brexit referendum.
Mr. Johnson, who was the mayor of London at the time, wrote a newspaper column about how Mr. Obama replaced a bust of Winston Churchill in the Oval Office with one of Martin Luther King Jr. He attributed the switch to “the part-Kenyan president’s ancestral dislike of the British Empire.”
That line stung Mr. Obama, according to his former aide, Benjamin J. Rhodes, who said later that Mr. Obama viewed it as “racially tinged.” Two former British diplomats said they worried that some in Mr. Biden’s circle — which draws heavily on people who worked for Mr. Obama — believed Mr. Johnson should pay a price for what he had said about the former president.