Trump is scheduled to speak at 10:00 a.m. (ET). Watch live video below:
Trump has signaled he will sign the massive spending measure keeping federal agencies operational through September 30.
But it does not specifically fund a wall on the US-Mexico border, and Trump's emergency declaration would help him bypass Congress to locate and redirect billions of dollars in funding to build it.
Trump will take the action "to ensure we stop the national security and humanitarian crisis at the border," White House Press Secretary Sarah Sanders said shortly before the Senate passed the spending bill.
The House of Representatives followed suit hours later, and Trump has until a midnight Friday deadline to sign it and prevent the government from going into shutdown for the second time this year.
Signing the spending bill would bring an end to a rolling, two-month battle over government funding. But by declaring an emergency, Trump opens a new confrontation -- and creates some of the riskiest legal peril of his term.
House Speaker Nancy Pelosi, Trump's Democratic nemesis in Congress, said members of her caucus were "reviewing our options" about how to respond to Trump's move.
"Declaring a national emergency would be a lawless act, a gross abuse of the power of the presidency and a desperate attempt to distract from the fact that President Trump broke his core promise to have Mexico pay for his wall," Pelosi and Senate Minority Leader Chuck Schumer said in a statement.
Senate Majority Leader Mitch McConnell said he backs the president, but several others in the Republican camp have expressed deep reservations.
"I have concerns about the precedent that could be set with the use of emergency action to re-appropriate funds," veteran Republican Senator Chuck Grassley said in a statement.
Senator Susan Collins said it "would be a mistake on the part of the president" to declare such an emergency, warning it would "undermine" lawmakers' all-important role as financial appropriators.
Many lawmakers have said they have no idea from where Trump will draw the funding. Democrats in particular have signaled that the move would open the door to future presidents declaring emergencies on various topics, from gun violence to climate change to the opioid crisis.
- Court challenges likely -
Under the National Emergencies Act, the president can declare a national emergency, providing a specific reason for it.
That allows the mobilization of hundreds of dormant emergency powers under other laws, which can permit the White House to declare martial law, suspend civil liberties, expand the military, seize property and restrict trade, communications and financial transactions.
Trump has repeatedly stated he will look to other federal sources to fund his wall, one of his top campaign trail promises from 2016.
There is broad expectation that Trump's move would be challenged in court.
And House Judiciary Committee Jerry Nadler expressed support for a joint congressional resolution to "terminate" Trump's emergency declaration.
That resolution would likely pass the Democratically-controlled House, but it would face a tough road in the Senate, where several Republicans may not wish to cross the president.
Even if it passed, Trump could veto it.
The spending measure includes only $1.375 billion for border barriers or fencing, far from the $5.7 billion that Trump has sought for his long-promised border wall -- a demand that led to the recent 35-day government shutdown, the longest in US history.
The compromise package funds until September 30 the 25 percent of the government whose operations would lapse if the bill is not signed by Trump by the midnight Friday deadline.
Pelosi expressed disdain for the president's upcoming emergency declaration, warning that he was doing "an end run around Congress."
"It's not an emergency, what's happening at the border," she said in comments echoed by multiple Democratic lawmakers.
- Conservative ire -
Trump is widely seen to have suffered politically more than Democrats over the previous shutdown fight.
But he also needs to assuage conservatives livid that the president retreated from his wall-funding demand, a longtime campaign pledge.
"This bill must NOT be signed by @realDonaldTrump," tweeted Fox News host Laura Ingraham, warning it would serve as a "stimulus" for illegal immigration in part because it puts limits on law enforcement's ability to deport some undocumented immigrants.
The bipartisan spending measure, negotiated for weeks, notably does not contain the word "wall" in its 1,165 pages, instead calling the structures physical barriers and pedestrian fencing.
