It follows warnings Congress will refuse to ratify any new trade agreement with the UK if they think the Government is undermining the Northern Ireland peace process. Boris Johnson has been threatening to overwrite parts of the EU withdrawal agreement arguing they infringe on British sovereignty.
However, critics claim this could undermine the 1998 Good Friday agreement which brought to an end decades of sectarian conflict.
During his visit Mr Raab is expected to meet US Secretary of State Mike Pompeo and Nancy Pelosi, the Democrat Speaker of the House of Representatives.
Brexit is expected to feature heavily after the government announced plan to unilaterally alter the withdrawal agreement.
Under the terms of the deal Britain agreed with the EU Northern Ireland was to remain very closely aligned to the European single market.
Dominic Raab is in Washington for talks with US politicians
Boris Johnson pictured outside Parliament on Tuesday
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As a result, the agreement mandated customs checks on some goods travelling between Northern Ireland and the rest of Britain.
The Government argues this infringes UK sovereignty and could prevent the export of food to Northern Ireland from the rest of the country.
It’s Internal Market Bill, which partially overwrites the deal struck with Brussels, passed its first two House of Common’s readings on Monday.
After the Government announced its plans last week Ms Pelosi warned it could block any UK-US trade deal.
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Nancy Pelosi has warned against undermining the Brexit withdrawal agreement
She said: “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.
“The UK must respect the Northern Ireland Protocol as signed with the EU to ensure the free flow of goods across the border.
“If the UK violates that international treaty and Brexit undermines the Good Friday accord, there will be absolutely no chance of a US-UK trade agreement passing the Congress.
“The Good Friday Agreement is treasured by the American people and will be proudly defended in the United States Congress.”
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The Brexit withdrawal agreement ties Northern Ireland closely to the European single market
Currently the Democrats have a majority in the House of Representatives, the lower chamber of Congress.
Separately, on Tuesday a number of senior Congressmen signed a joint letter warning any changes to the withdrawal agreement could have “disastrous consequences for the Good Friday Agreement and broader process to maintain peace on the island of Ireland”.
They added: “We therefore urge you 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.”
In response 10 Downing Street said it would ensure the Good Friday Agreement “is upheld in all circumstances and harmful defaults do not inadvertently come into play which could jeopardise the huge gains of the peace process”.
The PM’s office continued: “We are absolutely committed to no hard border and no border infrastructure between the Republic of Ireland and N Ireland.
“This is about a legal safety net and not having east-west checks suddenly imposed which run directly counter to Belfast (Good Friday) Agreement.
“We will continue to engage with our US partners on a bipartisan basis to ensure that our positions are understood and to ensure we agree a trade deal with widespread support in the US.”
Brexit supporters celebrate the UK’s EU withdrawal on January 31
Whilst Britain as a whole voted for Brexit in June 2016 a majority in Northern Ireland voted remain.
The UK finally left the EU on January 31 following a number of delays caused by parliamentary deadlock.
The current transition period, during which Britain remains tied to many EU rules and regulations, is due to expire at the end of this year.