The news of Trump's decision came via Senate Majority Leader Mitch McConnell, who said he would drop his opposition to the national emergency move in order to advance the government funding measure.
Speaking on the Senate floor Thursday, McConnell sought to reassure lawmakers unsure of the President's position before taking a vote on the plan, which falls short of providing the $5 billion in border wall funding Trump had demanded.
"He has indicated he is prepared to sign the bill. He will also be issuing a national emergency declaration at the same time," McConnell said. "I've indicated to him that I'm going to support the national emergency declaration. So for all of my colleagues, the President will sign the bill. We will be voting on it shortly."
It provided reassurance amid questions about the President's support for the deal, which was struck by a bipartisan panel of negotiators. Aides had said earlier Thursday they were concerned Trump would reject the spending compromise -- a major shift from earlier this week, when officials indicated privately that he would.
McConnell's abrupt announcement Thursday that Trump would sign the spending package -- ahead of any official word from the White House on the President's position -- came after a day of consternation among Republican lawmakers and administration officials about whether the President would sign the bill.
The President's only public message was a midday tweet indicating he was still mulling the final text of the bill with his team at the White House. Even after McConnell's announcement, the White House was scrambling to make Trump's intentions official.
"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," press secretary Sarah Sanders wrote in a statement 25 minutes after McConnell spoke. "The President is once again delivering on his promise to build the wall, protect the border, and secure our great country."
Last-minute drama
In the hours leading up to the vote, the President phoned GOP allies on Capitol Hill to ask their advice and vent at some of the bill's shortcomings, leading many to believe he was backing away from his earlier support of the compromise spending legislation, according to people familiar with the calls. Trump told multiple allies he was considering not signing the bill.
Concern over the measure's contents extended to the White House, where aides spent all morning trying to digest the details of the 1,100-page bill and flag potential snags both to the President and to Capitol Hill. In briefings about the bill, the President expressed concern that something might be found buried in the bill after he signed it, leading to embarrassment.
He huddled with his acting chief of staff Mick Mulvaney, Homeland Security Secretary Kirstjen Nielsen and legislative affairs director Shahira Knight in the Oval Office on Thursday afternoon, as they intensely lobbied the President to not back away from the bill.
Amid the wrangling, many of the President's senior advisers stressed he should sign the package to avoid another government shutdown, which they said would damage him politically. They said signing a national emergency declaration or some other type of executive action would blunt whatever blowback he received from conservatives.
It wasn't immediately clear which path Trump would choose to secure border wall funding through a national emergency declaration. White House aides have said they expect any unilateral action to secure the funding to be met with legal challenges, and McConnell had said previously he would oppose such a move, citing presidential overreach.
He dropped that opposition on Thursday after weeks of remaining staunch in his position, revealing just how worried he was over Trump's support.
But Democratic congressional leaders said they could bring legal challenges.
"I may," said House Speaker Nancy Pelosi who asked about the prospect of challenging Trump in court. "That's an option and we will review our options."
"The President is doing an end run around Congress," Pelosi added.
Advisers said Trump had grown increasingly concerned about what is contained in the 1,100-page legislation that was released late Wednesday evening.
As more details about the package emerged, conservative figures in Trump's orbit voiced new displeasure at the bill. That included Fox host Laura Ingraham, who tweeted earlier Thursday that Trump should not sign it. The White House had attempted earlier this week to bolster support among Trump's media allies.
White House officials have been digesting the text since early morning and have briefed the President as they go along. The President tweeted midday he was "reviewing the funding bill with my team."
"1,000 pages filed in the in middle of the night take a little time to go through," one White House official told CNN's Jim Acosta.
Prayers?
Lawmakers were set to begin voting on Thursday afternoon. Many expressed hope -- even prayers -- the President would ultimately approve it.
"I pray" Trump signs the bill, said Sen. Richard Shelby, the Republican chairman of the Appropriations Committee. He said he spoke with Trump Wednesday night and the President was in "good spirits."
Other Republicans said they were hoping for assurances Trump will sign the bill if it gets to his desk, possibly later on Thursday.
"We'd like to know it's a bill the President is going to sign. Hopefully they will let us know," said Sen. John Thune, the second-ranking GOP leader as he left a Republican conference lunch where the issue was discussed at length.
Others Republicans said they were still parsing the legislative text before committing to supporting the plan.
Earlier this week, Trump had signaled to advisers and allies he was inclined to sign the bipartisan deal to avoid another government shutdown, and would use executive action to attempt securing additional border wall dollars.
"I think the President's evaluating what's in the bill. He's also evaluating the authority he has and I know he'll be making a decision before the deadline," Vice President Mike Pence, who is traveling in Warsaw, told reporters Thursday.
Private griping
In conversations with allies over the past days, Trump has griped that Republican negotiators were outplayed by their Democratic counterparts, securing a border funding number far smaller than Trump has spent the last two months demanding.
Privately, Trump has cast GOP's dealmaking efforts as inadequate and wondered why he, an experienced dealmaker, wasn't consulted at more regular intervals as the two sides haggled over an agreement. The White House acted largely on the sidelines while congressional negotiators struck a deal.
That was intentional, according to people familiar with the process, who noted Trump's attempts at brokering an agreement between lawmakers proved futile during the record-length government shutdown that ushered in the new year.
To appease the President, aides and some Republican lawmakers have cast the smaller figure, around $1.375 billion, as a down payment that will eventually lead to new wall construction.
Initially, Trump was distressed when he watched Sean Hannity and other Fox News hosts deride the plan, including as he watched recorded versions of prime-time programming during a late-night flight home on Monday from Texas, where he'd held a campaign rally.
After phone calls from the White House, some of Trump's allies took a softer approach, saying the deal was palatable as long as Trump went ahead with unilateral action to secure some funding for the border wall.
On Thursday, however, some of those voices returned to their initial skepticism.
"So the president has his hand forced to sign a 1,159 page bill that we KNOW is filled with amnesty, PORK and wiggle room? Total SCAM! @realDonaldTrump wasn't elected for this," Laura Ingraham wrote on Twitter. "This bill must NOT be signed by @realDonaldTrump."
CNN's Jim Acosta, Phil Mattingly and Manu Raju contributed to this report.
