Democratic House Judiciary Chair Jerry Nadler said in a statement that he would "support the enactment of a joint resolution to terminate the President's emergency declaration", and that he plans "to pursue all other available legal options". Nita Lowey, D-N.Y., chairwoman of the House Appropriations Committee, filed the 1,159-page bill that will keep money in federal coffers until the end of the fiscal year on September 30.
The declaration, should Mr Trump carry it out, will prevent a second bruising government shutdown but set the President and Congress up for further clashes down the road.
Shortly after the assurance, the Senate passed the legislation, which includes US$1.375 billion (S$1.86 billion) for about 90km of fencing along the US-Mexico border, about a quarter of the US$5.7 billion the President has asked for to build the wall that was a centrepiece of his election campaign.
A possible target for fattening border wall funding would be the billions of dollars appropriated this year for military construction.
Congress has the ability, under the act, to overrule a presidential declaration. Trump's frustration with that figure is why he has chose to declare a national emergency to give himself the flexibility to divert other funds for construction of a barrier.
Ms Pelosi has not indicated whether House Democrats planned to legally challenge the president.
But he said that federal judges might be reluctant to get involved, especially because Congress has the authority under the National Emergencies Act to terminate a national emergency. The bill must be signed into law by Friday to keep the government from running out of funds and suspending some operations.
Pelosi accused Trump of doing "an end-run" around Congress and around the Constitution's separation of powers that gives Congress, not the president, federal spending authority.
Democrats immediately sought to discredit Trump calling for a national emergency by suggesting that there is no "emergency" on the border, despite the hundreds of thousands of people who are apprehended every year illegally trying to enter the USA on the southern border.
After a 35-day partial government shutdown over that issue that stretched from late December to late January, a House-Senate conference committee was formed to negotiate the final details on border security spending.
Individuals or businesses with contracts canceled because of a redirection of military funds might be better placed to challenge the president in court, as would private landowners whose property might be seized, Chesney said.
Other Republicans said they were hoping for assurances Mr Trump will sign the bill if it gets to his desk, possibly later on on Thursday.
Trump has repeatedly stated he will look to other federal sources to fund his wall, noting Wednesday that the administration has access to "a lot of money" being repurposed from existing federal funds to cover unmet wall construction costs.
The money is immediately available for use by the president, the aides said, but it will take some time to for him to spend it. Building a wall across the southern border will require lengthy negotiations with private landowners, they noted.
"A Democratic president can declare emergencies as well", Pelosi said. The legislation also includes more funding for some kind of barrier at the border.