Trump's signature on a 1,159-page bill will prevent another partial government shutdown at midnight Friday, just three weeks after a record 35-day closure shuttered about a quarter of U.S. government operations and furloughed 800,000 federal civil servants.
The legislation will hand Trump $1.375 billion in funding for a barrier along about 90 kilometers of the 3,200-kilometer border, less than a quarter of the $5.7 billion he wanted for a wall. He said this week he was "not thrilled" with the lesser amount a bipartisan group of lawmakers agreed on.
By declaring the national emergency, however, Trump plans to try to circumvent Congress by tapping unused federal funds to construct the wall, an action sure to invite a legal challenge from opposition Democrats and other groups. The dispute could be tied up in courts for a lengthy period of time, possibly delaying the actual construction of the wall.
Trump has long vowed to build the border wall, saying it was necessary to block caravans of migrants coming from Central America, stop illegal drugs at the border and keep criminals from entering the United States. It was his most enduring pledge from his successful 2016 campaign for the White House, with his most ardent supporters often shouting, "Build the wall!"
White House spokeswoman Sarah Sanders said, "President Trump will sign the government funding bill, and as he has stated before, he will also take other executive action — including a national emergency — to ensure we stop the national security and humanitarian crisis at the border. The president is once again delivering on his promise to build the wall, protect the border and secure our great country."
Trump's decision to sign the legislation came as Congress voted on the funding deal to prevent another government shutdown. While congressional leaders called for Trump to sign the bill, he had not, until Thursday afternoon, said what he would do, tweeting that he was reviewing the legislation.
Senate Majority Leader Mitch McConnell disclosed Trump's intentions to declare a national emergency, shortly before the Senate approved the legislation that would provide funding for several major U.S. agencies through September and include the barrier money. The House of Representatives was also expected to approve the legislation Thursday before sending it to Trump, whose signature is needed for it to become law.
McConnell told the Senate, "I've just had an opportunity to speak with President Trump, and he has indicated he's prepared to sign the bill. He will also be issuing a national emergency declaration at the same time. And I've indicated to him that I'm going to support the national emergency declaration."
But Senate Democratic leader Chuck Schumer, said, "There is word the president will declare a national emergency. I hope he won't. That would be a very wrong thing to do."
Speaker Nancy Pelosi, the leader of the Democratic-controlled House, also assailed Trump's decision to declare a national emergency, warning that what he was doing amounted to "an end run around Congress. It's not an emergency, what's happening at the border."
Members of Congress and their staff worked late Wednesday to finalize the legislation crafted by a bipartisan committee tasked with finding a border security agreement.
Aside from the barriers, the legislation also includes technology upgrades for screening at border entry points, more customs officers and humanitarian aid.
"As with all bipartisan agreements, it's a product of compromise," Schumer said this week. "Each side gave a little, each side got a little."
Before the last shutdown, which started Dec. 22, Trump had forecast he would sign a short-term funding measure to keep the government open while border security negotiations continued, but he had changed his mind.
"The president must not repeat his mistakes of the recent past. President Trump, sign this bill," Schumer said.
As he considered whether to sign the bill, Trump said, "As we review the new proposal from Congress, I can promise you this, I will never waver from my sacred duty to defend this nation and its people."
Under Trump, Congress has not authorized any funding for a wall. But wall repairs and replacements for deteriorating sections along the border have been ongoing.