Nancy Pelosi, the Democrat House speaker, said she may launch a legal challenge and warned the move would create “great unease and dismay” among her party.
“It is not an emergency, what is happening at the border,” Ms Pelosi said, calling the claim an “illusion”.
She insisted that the constitution gave the “power of the purse” to Congress, appearing to question whether Mr Trump was overstepping his powers.
Mr Trump has repeatedly stated he will look to other federal sources to fund his wall, noting on Wednesday that the administration has access to “a lot of money” being repurposed from existing federal funds to cover unmet wall construction costs.
But some Republicans have been warning that an emergency declaration would set a potentially problematic precedent.
“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.
“Accordingly, I will study the president’s declaration closely.”