The president invited cameras into the Oval Office to record the moment, saying, "As president, protection of the American people is my highest priority."
Surrounded by law enforcement officials and families of those allegedly killed by undocumented immigrants, he rejected the resolution as "reckless," calling the situation with illegal immigration a "tremendous emergency" and a "tremendous crisis."
"People hate the word invasion, but that's what it is," Trump said.
Attorney General William Barr was on hand to tell the president that his emergency declaration was "clearly consistent with the law."
Homeland Security Secretary Kirstjen Nielsen told him that the fact that it was an emergency was "undeniable."
The president's veto was in response to a resolution that passed the Senate Thursday with the support of 12 Republican senators, who joined Democrats in voting down the president's action as executive overreach. The House had previously passed the measure.
Shortly after the president issued his veto, Senate Democratic Leader Chuck Schumer issued a statement saying the president had chosen to "trample on the Constitution."
“It is no surprise that the president holds the rule of law and our Constitution in minimal regard. There is no emergency; Congress has refused to fund his wall multiple times; Mexico won’t pay for it; and a bipartisan majority in both chambers just voted to terminate his fake emergency," Schumer said.
“While the president has chosen to trample all over the Constitution, we Democrats in the Senate will never stop defending our country from an overreaching president,” he said.
House Speaker Nancy Pelosi called it "a power grab."
“The House and Senate resoundingly rejected the President’s lawless power grab, yet the President has chosen to continue to defy the Constitution, the Congress and the will of the American people," she said in a statement.
“On March 26, the House will once again act to protect our Constitution and our democracy from the President’s emergency declaration by holding a vote to override his veto," she said. “House Republicans will have to choose between their partisan hypocrisy and their sacred oath to support and defend the Constitution.”
Even though the resolution passed both the House and Senate, it's unlikely there would be enough votes to override his veto.
Regardless, the president and White House had engaged in a behind-the-scenes lobbying effort of Senate Republicans in the days before the Senate vote, which amounted to a stunning rejection of the president in his effort to advance one of his central policy priorities.
Within minutes of the Senate vote, he sent a one-word tweet: "VETO!"
Prior to that vote, the president seemed nonchalant about the prospect of having the issue his first veto but also accused Republicans of "overthinking" the issue as he sought to boil it down to a matter of border security.
"I'll probably have to veto. It won't be overturned and the legal scholars say it's totally constitutional," Trump said on Thursday. "It is very important. It is a border security vote. It is pure and simple, a vote for border security."
On Friday, he tweeted that he'd "like to thank all of the Great Republican Senators who bravely voted for Strong Border Security and the Wall."