WASHINGTON — President Donald Trump issued the first veto of his presidency on Friday, overruling Congress to protect the emergency declaration he’d used to circumvent lawmakers to build his border wall.

Flanked by law enforcement officials as well as the parents of children killed by people in the country illegally, Trump maintained that he is not through fighting for his signature campaign promise, which stands largely unfulfilled 18 months before voters decide whether to grant him another term.

“Congress has the freedom to pass this resolution,” Trump said, “and I have the duty to veto it.”

A dozen defecting Republicans joined Senate Democrats in approving the joint resolution on Thursday, which capped a week of confrontation with the White House as both parties in Congress strained to exert their power in new ways. It is unlikely that Congress will have the two-thirds majority required to override Trump’s veto, though House Democrats will try nonetheless.

Despite the sharp rebuke from Congress, Friday’s event had the victory lap feel of a bill-signing ceremony. Trump was surrounded in the Oval Office by supporters who offered profuse thanks and frequent applause. After many had spoken, Trump dramatically signed his veto message and then held the document up for the cameras to capture.

He then distributed pens as mementos of the occasion.

Trump wants to use the emergency order to divert billions of federal dollars earmarked for defense spending toward the southern border wall. It still faces several legal challenges from Democratic state attorneys general and environmental groups who argue the emergency declaration was unconstitutional.

Many lawmakers said the vote was not necessarily a rejection of the president or the wall, but protections against future presidents — namely a Democrat who might want to declare an emergency on climate change, gun control or any number of other issues.