(CNN) Some members of the US Congress have issued words of advice for the United Kingdom as it continues Brexit negotiations: Anything resembling a hard border between Northern Ireland and the Republic of Ireland, an EU member, will not be looked upon kindly.
Irish Deputy Prime Minister Simon Coveney's visit to Washington this week highlighted the problems the United Kingdom could face with future trade deals should members of Congress find issue with how the Brexit process plays out.
In an interview with CNN on Wednesday, Rep. Brendan Boyle, an Irish-American and member of the "Friends of Ireland" caucus that met with Coveney, said a hard border arising from Brexit negotiations would have a "major impact" on Capitol Hill.
"There's absolutely no way the US is going to do anything that would sacrifice our relationship with the EU," said Boyle, a Democrat from Pennsylvania. Congress would need to approve any future US-UK trade deals, he said.
"It's 2019 -- it's not the 1800s," he added. "And I think some people maybe sometimes forget that."
Boyle said the group of 15 Congressional members that met with Coveney agreed on the need for a backstop -- a sort of insurance policy designed to avoid a hard border -- saying it "is paramount" and something they take "deadly serious."
UK Prime Minister Theresa May has said that she is committed to delivering a Brexit that avoids a hard border. Such a border would involve checkpoints, cameras and border posts -- infrastructure that was often targeted by Irish nationalist paramilitaries during the bloody 40-year sectarian conflict known as "The Troubles."
A soft border -- where goods, people and vehicles freely pass through a seamless border between Northern Ireland, which is part of the UK, and the Republic of Ireland -- is an integral part of the peace accord known as the Good Friday Agreement.
Several powerful members of Congress have a vested interest in the border position. Rep. Richard Neal, a Massachusetts Democrat and chairman of the "Friends of Ireland" caucus, has long worked to help progress peace in Northern Ireland and was part of the group that met with Coveney. He is also the chairman of the House Ways and Means Committee, which will oversee any future trade deal between the United Kingdom and United States.
At one of the Coveney events this week, Rep. Peter King, a Republican from New York, said bluntly: "If the British want to consider any type of trade agreement with the US, it's important that that soft border be maintained."
The backstop has been a particularly thorny issue as May negotiates with both the British Parliament and the European Union.
But British politicians have been firm that they will not back her deal without changes. Last month, the Parliament voted to reopen the withdrawal agreement and seek alternative arrangements to the backstop.
The night of that UK parliamentary vote last month, Boyle introduced a resolution in the House of Representatives opposing a hard border.
Boyle said the move was based on seeing "the British government renege on the backstop it had helped negotiate, that it had in part asked for and agreed to."
He added, "That was deeply concerning to me. I wanted to show that there were a number of us on Capitol Hill that were paying attention to this."
The United Kingdom is scheduled to leave the European Union on March 29.