The prospect of a U.S.-U.K. trade deal is facing potential hurdles both in Congress and on the campaign trail, as lawmakers and Democratic nominee Joe Biden are threatening to slam the brakes on any deal – over concerns that a brewing Brexit fight could imperil a 1998 Northern Ireland peace agreement.

“We can’t allow the Good Friday Agreement that brought peace to Northern Ireland to become a casualty of Brexit,” Biden tweeted on Wednesday. “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.”

BREXIT DRAMA IN THE UK AS BORIS JOHNSON PUSHES BILL TO OVERRIDE EU EXIT DEAL

U.S. and U.K. officials have been working on a free trade deal for when the U.K. finalizes its departure from the E.U., which Brits voted for in 2016, at the end of the year. As an E.U. member, the U.K. was barred from making separate trade deals, and a U.S.-U.K. deal is seen as vital for the U.K. in its post-Brexit future.

But negotiations between the E.U. and the U.K. for its own trade deal have stalled, leading to U.K. fears that the E.U. will use last year’s Withdrawal Agreement – which included a protocol to keep an open border between Ireland (an E.U. member) and Northern Ireland (part of the U.K.) – to impose a hard trade barrier between Northern Ireland and Britain.

“We are being told that the E.U. will not only impose tariffs on goods moving from Great Britain to Northern Ireland, but that they might actually stop the transport of food products from GB to NI,” Prime Minister Boris Johnson said in an op-ed on Saturday.

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As a result, Johnson’s government has introduced an Internal Market Bill, which would allow ministers to override parts of the Withdrawal Agreement. It has led to fears in Europe and Washington that it risks bringing instability to Northern Ireland by erecting a border between Ireland and Northern Ireland, putting the 1998 Good Friday agreement in peril.

House Speaker Nancy Pelosi, D-Calif., sounded the alarm last week and said there would be “no chance” of a vital U.S.-U.K. trade deal if the U.K. were to undermine the agreement. She had made a similar threat last year.

“Whatever form it takes, Brexit cannot be allowed to imperil the Good Friday Agreement, including the stability brought by the invisible and frictionless border between the Irish Republic and Northern Ireland,” Pelosi said in a statement. “The U.K. must respect the Northern Ireland Protocol as signed with the EU to ensure the free flow of goods across the border.”

British Foreign Secretary Dominic Raab speaks at a press conference with Secretary of State Mike Pompeo at the State Department, Wednesday, Sept. 16, 2020 in Washington. (Nicholas Kamm/Pool via AP)

Meanwhile, members of the House Foreign Affairs Committee wrote to Johnson, urging him “to abandon any and all legally questionable and unfair efforts to flout the Northern Ireland protocol of the Withdrawal Agreement and look to ensure that Brexit negotiations do not undermine the decades of progress to bring peace to Northern Ireland and future options for the bilateral relationship between our two countries.”

PELOSI WARNS 'NO CHANCE' OF US-UK TRADE DEAL IF BREXIT UNDERMINES GOOD FRIDAY ACCORD 

Those members – Chairman Eliot Engel, D-N.Y.; Richard Neal, D-Mass.; Bill Keating, D-Mass., and Peter King, R-N.Y. – reiterated Pelosi’s conditions for a deal and added that “if these reported plans were to go forward, it would be difficult to see how these conditions could be met.” Any deal would need House approval to be ratified.

The Trump administration appears less concerned than Biden and the members of Congress, with Secretary of State Mike Pompeo on Wednesday saying that while it had made clear the importance of the Good Friday Agreement, the administration trusts the U.K. to “get it right.”

“We know the complexity of the situation. We’ve done what we can to provide assistance where we can,” Trump said at a press conference alongside U.K. Foreign Secretary Dominic Raab. “In the end, this will be a set of decisions with respect to this that the United Kingdom makes, and have great confidence that they will get this right in a way that treats everyone fairly and gets a good outcome for what it was the people of the United Kingdom voted for several years back.”

Raab, meanwhile, said he had “very positive discussions” with both the administration and with members of Congress – and would later be meeting with Pelosi. He said the threat to the Good Friday Agreement came from “the E.U.’s politicization of the issue.”

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Other British lawmakers reacted with more hostility, however. Former Conservative Party leader Iain Duncan Smith told the Times of London that Biden should not lecture the U.K.

“We don’t need lectures on the Northern Ireland peace deal from Mr. Biden,” he said. “If I were him I would worry more about the need for a peace deal in the USA to stop the killing and rioting before lecturing other sovereign nations.”