Democratic and Republican politicians have sharply criticised President Trump's plan to use emergency powers to pay for a border wall with Mexico.
Trump is due to declare the emergency shortly in an attempt to bypass Congress, which has refused to approve $5.7bn (£4.4bn) for the wall.
Senior Democrats accused the president of a "gross abuse of power". Several Republicans also voiced concern.
Building a border wall was a key pledge in Trump's election campaign.
Declaring a national emergency would give Trump access to billions of dollars for his project.
The president agreed on Thursday to sign a spending bill that does not include finance for the wall. Disagreement over the issue led to a 35-day government shutdown early this year - the longest in US history.
The spending bill is due to be signed shortly to avert another shutdown. Citing unnamed White House officials, US media outlets reported that the president would sign the emergencies act at the same time.
Can Congress stop Trump's emergency move?
The National Emergencies Act contains a clause that allows Congress to terminate the emergency status if both houses vote for it - and the president does not veto.
With a comfortable majority in the House, Democrats could pass such a resolution to the Senate. The Republicans control the Senate, but a number of Republican senators have been vocal in their unease about the president invoking a national emergency.
The dissenting Republicans include 2012 presidential contender and new senator for Utah Mitt Romney, Florida Senator Marco Rubio, and the senator from Maine Susan Collins, who said the move was of "dubious constitutionality".
The resolution would however still require Trump's signature to pass, allowing him to veto it. A supermajority in both houses of Congress is needed to overturn a presidential veto.
What did the White House say?
"The president is once again delivering on his promise to build the wall, protect the border, and secure our great country," White House Press Secretary Sarah Sanders said in a statement on Thursday.
She said Trump would "take other executive action - including a national emergency - to ensure we stop the national security and humanitarian crisis at the border".
The compromise legislation was approved in an 83-16 vote in the Senate on Thursday. The House of Representatives later also backed the measure, by 300 to 128.
The package includes $1.3bn in funding for border security, including physical barriers, but it does not allot money towards Trump's wall.
Speaking on the Senate floor on Thursday, Republican Majority Leader Mitch McConnell indicated his support for the president's national emergency move, saying the president was taking action with "whatever tools he can legally use to enhance his efforts to secure the border".
How have Democrats responded?
Speaker of the House Nancy Pelosi and Senate Democratic leader Chuck Schumer issued a strongly worded joint statement condemning the 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," read the statement.
"He couldn't convince Mexico, the American people or their elected representatives to pay for his ineffective and expensive wall, so now he's trying an end-run around Congress in a desperate attempt to put taxpayers on the hook for it."
Pelosi had already suggested that Democrats would mount a legal challenge.