House Speaker Nancy Pelosi warned Trump that if he issued a national emergency over border security, he would set the precedent for a future Democratic president to do the same when it comes to gun violence.
Despite widespread opposition in Congress to proclaiming an emergency, including by some Republicans, Trump is under pressure to act unilaterally to soothe his conservative base and avoid looking like he's lost his wall battle.
Facing a Friday deadline, congressional leaders came to an agreement earlier this week aimed at keeping the government open.
The bill would provide nearly $1.4 billion for 55 miles of additional border fencing identical to that already in place, far less than the $5.7 billion Trump had demanded before the 35-day government shutdown that began in December.
His authority to do so will nearly certainly be challenged in court. But she added, "First of all, it's not an emergency, what's happening at the border".
What is a national emergency?He can do so at will. And, according to Politico's Jake Sherman, he angrily declared, "You're rude!"
Declaring a national emergency is a rare step.
Republican Senator Lindsey Graham, who is in regular contact with the White House, said Trump was "inclined to take the deal and move on".
"Government shutdowns are awful for workers, our economy and our national security. Where is the urgency?" That's far less than the $5.7 billion Trump demanded from Congress, which had sparked the shutdown. "The president should never again harm all three by forcing a government shutdown to get his way", he said.
The source said White House lawyers had vetted the figures and believed they would withstand a legal challenge. These included overcoming her pledge to not fund the wall at all and rejecting a Democratic proposal for numerical limits on detaining some immigrants, said a Republican speaking on condition of anonymity to describe private conversations.
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Stephen Harper waived cabinet privilege during the Mike Duffy scandal and Paul Martin did the same during the Gomery inquiry into the federal sponsorship scandal.
Disputes over the constitutionality or legality of the exercise of emergency powers are judicially reviewable, the document prepared for Congress states.
Mr. Trump had long floated the possibility of a national emergency, saying he would probably move forward with one if Congress failed to provide him with enough border wall funding.
Democrats, who control the House, celebrated the measure as a win against Mr. Trump's agenda.
But, Trump hopes with the national emergency he can still follow through on his campaign promise.
An emergency declaration will mean funding is shifted from other federal priorities to ensure the wall is built. White House economic adviser Larry Kudlow, speaking to reporters at the White House on Thursday, said Trump now is "taking a look" at the measure.
The president maintains that the wall is necessary to stem irregular immigration and drug trafficking.
With polls showing the public blamed him and GOP lawmakers, Trump folded on January 25 without getting any of the wall funds.
Finally, voices counseling calm in the face of the emergency declaration point to the 59 emergency declarations made by presidents since the 1976 National Emergencies Act. "It's a humanitarian challenge to us", Ms Pelosi said. "He's trying an end-run around Congress in a desperate attempt to put taxpayers on the hook for it".
Top Senate Democrat Chuck Schumer accused Trump of a "gross abuse of the power of the presidency".
Using emergency powers, Trump could reallocate federal funds from other projects to use on building a barrier.