The DUP leader Sir Jeffrey Donaldson has said that Chuck Schumer needs to read some history books, after the US Senate majority leader urged the DUP to go back into Stormont.
The comments came at the annual Ireland Funds dinner in Washington, attended by Irish political leaders including Leo Varadkar and powerful US figures like House speaker Kevin McCarthy and Mr Schumer, the most senior politician in the Senate.
Mr Schumer used his address to encourage the DUP to endorse the Windsor Framework, the new UK-EU post-Brexit deal announced last month.
"Now the Windsor Framework has been announced," Mr Schumer said, "I sincerely hopes it clears the way for the DUP to join Sinn Fein in a power-sharing agreement."
"I say to all parties in the North, but especially the DUP, let's get to the people's business, the business of power-sharing and self-governing."
He continued that "as majority leader of the United States Senate which decides on treaties, I will not support any trade deal between the US and the UK if any settlement undermines the Good Friday Agreement".
Sir Jeffrey told Sky News: "I would urge the senator to read some history books. Maybe he'd learn a little bit more about what really happens and the reality of the situation."
The DUP says the Windsor Framework is a step in the right direction, but maintains that certain areas require change. The party is looking to Rishi Sunak to step in, and make the changes it requires before it will consider going back into power-sharing.
Please use Chrome browser for a more accessible video player
"We're saying the [UK] government needs to go further and to dig deeper to ensure that what we get not only works for here and now but is futureproof," Sir Jeffrey added.
The build-up to St Patrick's Day has seen the traditional exodus of Ireland's political leaders to the US.
Ahead of his meeting with Joe Biden on Friday, the Irish prime minister told Sky News that this is "a very sensitive moment in this process".
Asked if any changes can possibly be made to the new post-Brexit deal, Mr Varadkar said: "It's not for me to speak for other people. What's happened is that we have an agreement at long last between the UK and the European Commission that has been endorsed by the Irish government, also the UK government.
"Ultimately it's going to be a decision for the UK prime minister when they press ahead and what they decide to do with their own domestic legislation."
Expect delicate language from the Irish govt as the focus remains on DUP
Leo Varadkar has a reputation for occasionally getting in hot water with his forthright answers. But during this interview with Sky News, the Irish leader was restrained and disciplined when repeatedly questioned about what he wanted the DUP to do about the Windsor Framework.
An outside observer might think, weeks after the deal was heralded with jubilance by London and Brussels, that the Irish government might be ratcheting up the pressure on the largest unionist party to accept the deal, and get Stormont back working.
But Mr Varadkar's steadfast refusal to opine strongly reflects a belief in Dublin that Sir Jeffery needs time and space to embrace the deal. For now, the DUP's public stance is that - just like the protocol - real changes are needed to the text of the Windsor Framework.
Privately, many believe that the party's leader is keen to find a way to get back to power-sharing without completely dismissing the new agreement. But he needs to carry the hard-line wing of his party with him. And that task would be made significantly more difficult if he's perceived to be bowing to any pressure from south of the border.
So as the St Patrick's week events continue in the American capital, expect delicate language from the Irish government as the focus remains on the DUP. Whether Joe Biden will be as diplomatic on Friday is another matter.
The Irish leader declined to say what his message to Sir Jeffrey was, asserting his conversations would remain private.
Asked if it was a mistake by the British government to brand the agreement as the Windsor Framework, and involve King Charles in an overt attempt to woo unionists, Mr Varadkar said that Dublin had no objections at the time.
The Taoiseach said that "we were asked whether we were comfortable about the term Windsor Framework being used and we said that we were".
"What's mattered to us all along is the substance," Mr Varadkar added.
Read more:
What is in the new post-Brexit deal on Northern Ireland?
Leo Varadkar says it's 'very important we listen to the DUP'
Click to subscribe to the Sky News Daily wherever you get your podcasts
He also spoke of his hopes for a good working relationship with Mr Sunak, who he has yet to meet in person.
"I do think he means business," Mr Varadkar said of the UK prime minster. "There's a real opportunity to restore British-Irish relations to where they were before Brexit. They were really good before Brexit."
It's expected the US president will give further details of next month's visit to the island of Ireland during Friday's bilateral meeting with the Taoiseach.
Mr Biden is also expected to reiterate his administration's support for the Windsor Framework, and encourage all parties in Northern Ireland to endorse it.