President Donald Trump has issued the first veto of his presidency, 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, Mr Trump maintained that he is not finished 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,” Mr 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.

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President Donald Trump looks at notes (Evan Vucci/AP)

It is unlikely that Congress will have the two-thirds majority required to override Mr Trump’s veto, though House Democrats have suggested they would try nonetheless.

Mr Trump wants to use the emergency order to divert billions of federal dollars earmarked for defence spending toward the southern border wall.

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Congress was overruled by the veto (J. Scott Applewhite/AP)

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 Mr 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.

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A boy rides his bike along a razor-wire-covered border wall that separates Nogales, Arizona, from Nogales, Mexico (Charlie Riedel/AP)

“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.

Mr Trump maintained that the situation on the southern border is “a tremendous national emergency”, adding, “our immigration system is stretched beyond the breaking point”.