Joe Biden, with his defense of the Northern Ireland peace accord, has issued a warning to Britain over Brexit and support for an EU bruised by Donald Trump while making it clear he would preside over the Atlantic alliance.
"We can't allow the Good Friday Agreement that brought peace to Northern Ireland to become a casualty of Brexit," the Democratic presidential nominee tweeted.
"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," Biden said.
British Prime Minister Boris Johnson has angered the European Union with a proposed law that would breach provisions of the agreement made with the EU in January for Britain's exit from the 27-nation bloc.
Watch: What is a no-deal Brexit and what are the potential consequences?
The Britain-EU agreement pledges to maintain an open border between Ireland and Northern Ireland and there are fears any reversal could endanger the 1998 peace accord known as the Good Friday Agreement.
Besides Biden, the proposed British law has sparked concern among Democratic lawmakers in Washington and put congressional approval of a US-UK trade deal in doubt.
Nancy Pelosi, the Democratic speaker of the House of Representatives, did not mince words.
"If the UK violates its international agreements and Brexit undermines the Good Friday accord, there will be absolutely no chance of a US-UK trade agreement passing the Congress," Pelosi said in a statement.
Eliot Engel, chairman of the House Committee on Foreign Affairs and two other committee chairmen with large Irish constituencies, wrote a letter to Johnson.
"An Ireland divided by a hard border risks inflaming old tensions that very much still fester today," they said.
"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," they added.
The tweet by Biden, who is of Irish origin and would be just the second Catholic president of the United States if he is elected in November, came as British Foreign Secretary Dominic Raab was in Washington seeking to reassure Britain's US ally.
"Our commitment to the Good Friday Agreement and to avoid any extra infrastructure at the border between the north and the south is absolute," Raab said on Wednesday alongside US Secretary of State Mike Pompeo.
In London, Johnson's spokesman said the bill being debated in the British parliament was intended "precisely to make sure that the Belfast (Good Friday) Agreement is upheld in all circumstances."
Pompeo for his part said he "had great confidence that they will get this right in a way that treats everyone fairly."
Trump, a fervent supporter of Brexit and Johnson and a frequent critic of the EU, has said that he wants to get a US-UK trade deal done quickly.
Nile Gardiner, a fellow at the conservative Heritage Foundation in Washington, accused Biden of "recycling the anti-British talking points of the European Commission.
"Brexit will not undermine the Good Friday Agreement," said Gardiner, a former aide to Britain's ex-prime minister Margaret Thatcher.
"The threats against a US/UK free trade deal from liberals in Washington should be a wakeup call for London," he said. "The American Left is hugely anti-Brexit, massively pro-EU and even hostile towards Britain."
Trump has accused Biden of being weak and standing up to Britain provides the Democrat with a chance to show he too can bang on the table -- even if it is with a nation with which the US enjoys a "special relationship."
It also doesn't hurt that the target in this case is the pro-Trump Johnson.
"Some of us have been arguing that a Biden admin should invest in the UK and EU relationships simultaneously and help both figure out their future relationship beyond trade," said Thomas Wright, a senior fellow at the Brookings Institution.
Johnson, by pursuing his current course on Brexit, will "dramatically weaken the case for this approach," Wright said.
"It is more likely that the US will invest in the EU relationship and the UK will be on the margins until it abides by its commitments on the Irish border," he said.