Photo: The Canadian Press

Snow falls on the Capitol as the partial government shutdown continues in Washington, Thursday night, Jan. 17, 2019.

She imperiled his State of the Union address. He denied her a plane to visit troops abroad.

The shutdown battle between President Donald Trump and House Speaker Nancy Pelosi is playing out as a surreal game of constitutional brinkmanship, with both flexing political powers from opposite ends of Pennsylvania Avenue as the negotiations to end the monthlong partial government shutdown remain stalled.

In dramatic fashion, Trump issued a letter to Pelosi on Thursday, just before she and other lawmakers were set to depart on the previously undisclosed trip to Afghanistan and Brussels. Trump belittled the trip as a "public relations event" — even though he had just made a similar warzone stop — and said it would be best if Pelosi remained in Washington to negotiate to reopen the government.

"Obviously, if you would like to make your journey by flying commercial, that would certainly be your prerogative," wrote Trump, who had been smarting since Pelosi, the day before, called on him to postpone his Jan. 29 State of the Union address due to the shutdown.

Denying military aircraft to a senior lawmaker — let alone the speaker, who is second in line to the presidency after the vice-president, travelling to a combat region — is very rare. Lawmakers were caught off guard. A bus to ferry the legislators to their departure idled outside the Capitol on Thursday afternoon.

The political tit-for-tat between Trump and Pelosi laid bare how the government-wide crisis has devolved into an intensely pointed clash between two leaders determined to prevail. It took place as hundreds of thousands of federal workers go without pay and Washington's routine protocols — a president's speech to Congress, a lawmaker's official trip — became collateral damage.

Pelosi spokesman Drew Hammill said the speaker planned to travel to Afghanistan and Brussels to thank service members and obtain briefings on national security and intelligence "from those on the front lines." He noted Trump had travelled to Iraq during the shutdown, which began Dec. 22, and said a Republican-led congressional trip also had taken place.

Trump's move was the latest example of his extraordinary willingness to tether U.S. government resources to his political needs. He has publicly urged the Justice Department to investigate political opponents and threatened to cut disaster aid to Puerto Rico amid a spat with the island territory's leaders.

Some Republicans expressed frustration. Sen. Lindsey Graham tweeted, "One sophomoric response does not deserve another." He called Pelosi's State of the Union move "very irresponsible and blatantly political" but said Trump's reaction was "also inappropriate."

While there were few signs of progress Thursday, Vice-President Mike Pence and senior adviser Jared Kushner dashed to the Capitol late in the day for a meeting with Republican Senate Majority Leader Mitch McConnell. And the State Department instructed all U.S. diplomats in Washington and elsewhere to return to work next week with pay, saying it had found money for their salaries at least temporarily.

For security reasons, Pelosi would normally make such a trip on a military aircraft supplied by the Pentagon. According to a defence official, Pelosi did request Defence Department support for overseas travel and it was initially approved. The official wasn't authorized to speak by name about the matter, so spoke on condition of anonymity.

The official said the president does have the authority to cancel the use of military aircraft.

Rep. Adam Schiff of California slammed Trump for revealing the closely held travel plans.

"I think the president's decision to disclose a trip the speaker's making to a war zone was completely and utterly irresponsible in every way," Schiff said.

Trump's trip to Iraq after Christmas was not disclosed in advance for security reasons.