US President Donald Trump has declared a national emergency so he can build a border wall with Mexico.
The move allows him to try to bypass the US Congress to get the money for its construction.
Critics say he's overstepping his powers.
US House Speaker Nancy Pelosi and Senate Democratic leader Chuck Schumer describe the declaration as "unlawful", adding that it is "over a crisis that does not exist".
"This is plainly a power grab by a disappointed president, who has gone outside the bounds of the law to try to get what he failed to achieve in the constitutional legislative process.
"The president's actions clearly violate the Congress's exclusive power of the purse, which our founders enshrined in the constitution", they say in a joint statement.
"The Congress will defend our constitutional authorities in the Congress, in the courts, and in the public, using every remedy available."
— Kayleigh McEnany 45 Archived (@PressSec45) February 15, 2019
Announcing the move outside the White House, Mr Trump said there are "tremendous amounts of drugs flowing into our country - much of it coming from our southern border."
Playing down the severity of the decision, he said similar declarations have been made by other presidents " many times before."
"There has rarely been a problem - they sign it but nobody cares," he said.
"They sign it for far less important things in many cases. We are talking about an invasion of our country with drugs; with human traffickers; with all types of criminals and gangs."