President Donald Trump issued the first veto of his presidency on Friday, overruling Congress to protect his emergency declaration for border wall funding.

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 have suggested they would try nonetheless.

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.

Those cases could block Trump from diverting extra money to barrier construction for months or longer. American Civil Liberties Union, which filed one of the cases, said the veto is meaningless — like the declaration in the first place.

"Congress has rejected the president's declaration, and now the courts will be the ultimate arbiter of its legality. We look forward to seeing him in court and to the shellacking that he will receive at the hands of an independent judiciary," said Executive Director Anthony Romero.

Trump said the situation on the southern border is "a tremendous national emergency," adding, "our immigration system is stretched beyond the breaking point."

Trump is expected to issue his second veto in the coming weeks over a congressional resolution seeking to end U.S. backing for the Saudi Arabian-led coalition fighting in Yemen. The resolution was approved in the aftermath of the killing of journalist Jamal Khashoggi in the Saudi Consulate in Istanbul.

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AP writers Alan Fram, Lisa Mascaro, Catherine Lucey and Colleen Long contributed.