WASHINGTON - A day after President Donald Trump canceled military flights for a planned congressional trip to Afghanistan, House Speaker Nancy Pelosi abandoned her plans to travel instead on commercial flights due to security concerns.

Drew Hammill, a spokesman for Pelosi, D-Calif., blamed the Trump administration in a written statement for disclosing news of the trip, which included several other House Democrats, and thus increasing the danger to lawmakers.

"After President Trump revoked the use of military aircraft to travel to Afghanistan, the delegation was prepared to fly commercially to proceed with this vital trip to meet with our commanders and troops on the front lines," Hammill said.

Overnight, he added, a new State Department threat assessment indicated "that the President announcing this sensitive travel had significantly increased the danger to the delegation and to the troops, security, and other officials supporting the trip. . . . This morning, we learned that the Administration had leaked the commercial travel plans as well."

The travel standoff comes as the partial government shutdown, the longest in American history, entered its 28th day.

Trump on Thursday informed Pelosi in a letter that he was postponing the originally planned trip due to the shutdown and called it a "public relations event." That move was in apparent retaliation after Pelosi told Trump in a Tuesday letter that she intended to reschedule his planned Jan. 29 State of the Union address due to security concerns prompted by the shutdown.

"Obviously, if you would like to make your journey by flying commercial, that would certainly be your prerogative," Trump wrote.

Members of Congress routinely travel around the world as part of their congressional business; that travel is frequently done on military planes and arranged by State Department protocol officers.

Such trips are typically kept secret for security reasons until lawmakers are safely back in the United States.

Hammill said the trip to Afghanistan would be indefinitely postponed "so as not to further endanger our troops and security personnel, or the other travelers on the flights." But he said that lawmakers would not be cowed in discharging their constitutional duties

“The United States Congress is a coequal branch of government in our system of checks and balances,” he said. “The Congress has a constitutional responsibility to conduct oversight in the war zone where our men and women in uniform are risking their lives every day.”