Republican senators who bucked the president said they did so to preserve congressional control over the government's purse strings.
"We had a war against a king in the American revolution," said Sen. Lamar Alexander, R-Tenn. "This would be the first time that a president has ever asked for a certain amount of money from Congress, Congress has refused to provide it, and then the president has declared a national emergency under the 1976 act and said, 'I'm going to spend the money anyway.'"
Alexander added that he shares the president's goal of border security, but warned allowing Trump to redirect money without authorization could set a dangerous precedent.
House Speaker Nancy Pelosi, D-Calif., announced soon after the veto that the House would attempt to override it, after returning from a recess later this month.
Republicans, Pelosi said, "will have to choose between their partisan hypocrisy and their sacred oath to support and defend the Constitution."
Recent months have seen a surge in unauthorized crossings of the U.S. border with Mexico. Last month, roughly 76,000 people tried, including 43,000 children and family members, many fleeing violence and poverty in Central America.
"People hate the word invasion, but that's what it is," Trump said Friday.
The large numbers have put a strain on immigration enforcement, although critics say a wall would do little to discourage the crossings since many of those apprehended at the border turn themselves in voluntarily.
The border wall project still faces a variety of legal challenges, and despite Trump's veto, House and Senate votes against the emergency declaration could carry some weight in the courts.
Trump insists he's on solid legal ground, however.
"I think actually a national emergency was designed for a specific purpose like this, so we have a great case," Trump said. "Ideally, they shouldn't even sue in this case, if you want to know the truth. They shouldn't even be suing, but they will because they always do."
On Friday, Barr also said the president's emergency order was "clearly authorized under the law."
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