(Adds quote, background)
WASHINGTON, Aug 19 (Reuters) - U.S. Senate Democratic leader Chuck Schumer said on Monday he would oppose any post-Brexit trade deal between the United States and Britain if it undermined the Good Friday agreement, which helped end three decades of violence in Northern Ireland.
The Good Friday pact also dismantled all physical border infrastructure between European Union member Ireland and Northern Ireland, which is part of Britain, guaranteeing that people and goods on either side can move freely.
"The notion that America would now endorse a policy or agreement that undermines the success of the Good Friday Agreement is profoundly counterproductive and risks exacerbating sectarian polarization and eroding self-determination - and unleashing the potential for violence that comes with that reality. This cannot be allowed to happen," Schumer wrote in a letter to U.S. Secretary of State Mike Pompeo.
The U.S. Trade Representative's Office last October notified Congress that it intends to negotiate a trade agreement with Britain under the Trade Priorities and Accountability Act of 2015.
Under such procedures, known as "fast-track," Congress would not be able to alter the pact and would only be able to hold a simple-majority vote on it.
U.S. President Donald Trump's Republican Party holds a narrow majority in the Senate, but a trade deal considered under such a procedure would not pass if a handful of Republicans and all of the Democrats in the chamber voted against it.
U.S. House of Representatives' Speaker Nancy Pelosi said last week that a U.S.-UK trade deal would languish in Congress if it facilitated a reinstatement of a hard border between Northern Ireland and Ireland.
The future status of the border between Northern Ireland and Ireland has been one of the knottier issues of Britain's promised exit from the EU, popularly known as Brexit, now scheduled for Oct. 31. (Reporting by Tim Ahmann; Writing by Makini Brice and David Lawder; editing by Jonathan Oatis)