Joe Biden on Wednesday joined the clamour of Democrats warning Boris Johnson not to let the Northern Ireland Good Friday agreement become a casualty of his Brexit talks. The foreign secretary, Dominic Raab, is in Washington trying to repair relations with pro-Irish Democrats amid concerns that the UK’s attempt to leave the EU on its own terms will undermine the Good Friday peace agreement. Democrats have warned that if the UK undermines the peace process or reinstates a hard border to secure Brexit on its terms, the UK’s chances of securing the required congressional support for a UK-US free trade deal will disappear. Biden, the Democrat presidential candidate, said in a tweet on Wednesday night: “We can’t allow the Good Friday Agreement that brought peace to Northern Ireland to become a casualty of Brexit. 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.” UK diplomats will be deeply disturbed that with fewer than 50 days to the presidential election, the UK government has contrived to set up a major political clash with the candidate currently given a 75% chance of becoming the next president in January 2021. Nancy Pelosi, the House Speaker and leading Democrat, also issued a warning to Raab after their meeting. Raab had earlier accused the EU of trying to politicise the Brexit withdrawal agreement, and insisted the UK had no desire to set up new infrastructure on the border between Northern Ireland and Ireland. Pelosi said she welcomed his assurances but that she had reiterated the message she said she had given to leaders in London last year. She added: “If the UK violates its international agreements and Brexit undermines the Good Friday accord, there will be absolutely no chance of UK-US free trade agreement passing the Congress. “The Good Friday agreement is valued by the American people and will continue to be proudly defended by the US Congress.” Pelosi met Raab with the chairman of the Congress ways and means committee, Richard Neal. The pro-Irish lobby in Washington is as politically influential as that in the UK, even though its economy and defence capabilities are no match for London’s. Pelosi’s warning shows how the UK efforts to renegotiate the Northern Ireland protocol is having wider repercussions across the Atlantic, and so risking free trade with the US, one of the major touted benefits of Brexit. The possible election of Joe Biden as president only increases that risk, but whichever candidate wins a free trade deal needs the support of two-thirds of Congress to pass. Earlier, Raab, standing alongside the US secretary of state, Mike Pompeo, had told reporters: “Our commitment to the Good Friday agreement, and to avoid any extra infrastructure at the border between the north and south [of the island of Ireland], is absolute. “To be absolutely clear, the UK action here is defensive in relation to what the EU is doing. It is precautionary – we have not done any of it yet – and is proportionate. “What we cannot have – and this is contrary to the Northern Ireland protocol and a risk to the Good Friday agreement – is the EU seeking to erect a regulatory border down the Irish Sea between Northern Ireland and Britain.” Pompeo said simply it was a matter for the UK government but “he had great confidence that they will get this right in a way that treats everyone fairly and get a good outcome for the people of the UK that voted for this several years back”. Pompeo struck an aggressive note on Iran, the other point of friction between the UK and the US, by saying the US would return to the UN next week to reimpose UN sanctions, adding that this would mean the arms embargo on Iran, due to be lifted in October, would become permanent next week. “We’ll do all the things we need to do to ensure that those sanctions are enforced,” he said when asked how the US would achieve that and what it would do to punish countries that did not reimpose the measures. A major pro-Irish lobby group, the ad hoc committee to protect the Good Friday agreement, has vowed to build “a green wall” to defend the agreement, citing a compelling statement from former prime ministers John Major and Tony Blair. Four senior congressmen in a joint statement warned the UK’s plans “could have disastrous consequences for the Good Friday agreement and the broader process to maintain peace on the island of Ireland”. They added: “Many in the Congress and the US consider the issue of the Good Friday agreement and a potential US-UK free trade agreement are inextricably linked.” They also said they supported Pelosi’s statement that she would not back any free trade agreement that undermined the broader Irish peace process. The quartet urged Raab “to abandon any or all legally questionable and unfair efforts to flout the Northern Ireland protocol of the withdrawal agreement”. The letter was signed by Neal, Eliot Engel, the chairman of the foreign affairs committee in Congress, William Keating, the chair of the House subcommittee on European affairs, and Peter King, the only Republican congressman in the quartet. All four represent seats with large Irish populations. Neal told the New York Times he had been given previous assurances by Raab about the Good Friday agreement, but said he had been blindsided about the latest proposed changes in the withdrawal agreement. The British position has not been helped by the conflicting government explanations for why the protocol is being changed or the news that the external legal advice commissioned by UK law officers came exclusively from pro-leave barristers.