LONDON (BLOOMBERG) - Mr Boris Johnson's supporters turned on Mr Joe Biden after the Democratic presidential nominee ruled out any UK-US trade deal if the Prime Minister's effort to rewrite the Brexit withdrawal agreement threatens peace in Ireland.

"We don't need lectures on Northern Ireland peace deals from Mr Biden," former Conservative Party Leader Iain Duncan Smith said in an interview.

"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."

On Wednesday (Sept 16), Mr Biden said any trade deal would be contingent on preventing the return of a hard border in Northern Ireland, something Mr Johnson's move to re-write sections of the Brexit deal threatens to bring about, risking more than two decades of peace.

"We can't allow the Good Friday Agreement that brought peace to Northern Ireland to become a casualty of Brexit," Mr Biden said on Twitter.

"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."

Indiana-born Tory MP Joy Morrissey told Mr Biden to "climb down off your moral high horse" before deleting the tweet, while close Johnson ally Conor Burns tweeted sarcastically at the former vice-president.

Mr Johnson has long touted a free-trade accord with the US as one of the key prizes of Brexit and Mr Biden's intervention underscores how the Prime Minister is struggling to contain the political fallout from his plan to unilaterally rewrite the divorce treaty he signed with the European Union less than a year ago.

The government said the legislation aims to stop the EU erecting trade barriers between mainland Britain and Northern Ireland if no agreement can be reached on future arrangements for the region.

On Thursday, a spokesman for the Prime Minister reiterated that the government is committed to avoiding any return of a hard border on the island of Ireland.

Yet it's clear Mr Johnson's position is not cutting through with some US politicians.

Mr Biden also shared a strongly-worded letter, signed by four senior members of Congress, which urged Mr Johnson to "abandon any and all legally questionable and unfair efforts" to breach its agreement with the EU.

"The United States Congress will not support any free trade agreement between the United States and the United Kingdom if the United Kingdom fails to preserve the gains of the Good Friday Agreement and broader peace process," the letter said.

"If these reported plans were to go forward, it would be difficult to see how these conditions could be met."

The Prime Minister will be more encouraged by the tone of President Donald Trump's administration.

Secretary of State Mike Pompeo said on Wednesday trade talks are progressing well, and that he backed the UK as a trustworthy partner despite the furore over Mr Johnson's Brexit plan.

"I am confident they'll get it right," Mr Pompeo said at a joint press conference with UK Foreign Secretary Dominic Raab in Washington.

"We know the complexity of the situation."

Still, the risks are real for Mr Johnson, especially with US presidential elections looming in the fall.

Speaking to Irish national broadcaster RTE on Thursday, Chairman of the US House Ways and Means Committee Richard Neal said Mr Raab had told him during their meeting the UK's Withdrawal Agreement is "open to interpretation".

In response, Mr Neal said he told Mr Raab there would be no new trade deal between the UK and US if the Good Friday Agreement is undermined.

The approach to the UK will be "trust, but verify", Mr Neal told RTE.