Donald Trump has denied a military aircraft for the most senior Democrat in the House of Representatives shortly before she was due to embark on a series of overseas trips.
The US president hit back at Nancy Pelosi, the House speaker, a day after she told him to delay his State of the Union address until after the government shutdown was over.
"Due to the Shutdown, I am sorry to inform you that your trip to Brussels, Egypt and Afghanistan has been postponed," Mr Trump wrote in a retaliatory letter on Thursday.
"In light of the 800,000 great American workers not receiving pay, I'm sure you would agree that postponing this public relations event is totally appropriate."
"We will reschedule this seven-day excursion when the Shutdown is over."
Mrs Pelosi, a 78-year-old congresswoman from California, has been at loggerheads with Mr Trump, 72, throughout the shutdown, trading barbs and attempting to pin the blame on each other’s party.
Mrs Pelosi cited security concerns over her calls for Mr Trump to delay his speech, saying that the Secret Service and Homeland Security - which both help keep the event safe – had been partly “hamstrung” by the lack of funding.
The State of the Union address is one of the most prominent Washington events of the calendar, seeing a president speak to almost every member of the capital’s political elite.
Every congressmen and senator is invited to the speech, delivered in the House, as well as Supreme Court judges, members of the president’s cabinet and foreign ambassadors.
Security is extremely tight, with the roads around the US Capitol closed off in the hours before the speech.
Mrs Pelosi wrote in her letter: “Sadly, given the security concerns and unless government reopens this week, I suggest that we work together to determine another suitable date after government has reopened for this address or for you to consider delivering your State of the Union address in writing to Congress on January 29.”
The move was an apparent attempt to force focus of the shutdown, the longest in US history, back onto Mr Trump while also denying the president a prominent stage from which to chastise Democrats over the impasse.
The US president has insisted he will sign no spending bill that would reopen government unless it includes $5.7 billion for construction of his Mexico border wall – a key campaign pledge.
The Democrats, who hold the majority in the House, have refused to give Mr Trump his $5.7 billion and insist they will not talk about immigration reform until the government is back open.
One sophomoric response does not deserve another. Speaker Pelosi’s threat to cancel the State of the Union is very irresponsible and blatantly political.— Lindsey Graham (@LindseyGrahamSC) January 17, 2019
But while the nation's two most powerful leaders appeared to be engaged in a game of Constitutional one-upmanship, some lawmakers were unimpressed.
Republican senator Lindsey Graham tweeted: "One sophomoric response does not deserve another."