President Trump faces legal challenges following his decision to use emergency powers to build a wall at the US border with Mexico.
California and New York have announced that they will take legal action to challenge its decision to bypass Congress and obtain funding for the project.
The construction of the wall was an essential commitment of Mr. Trump's campaign.
Democrats said it was "blatant abuse of power" and promised to challenge it "using all available remedies".
On Friday, Mr Trump signed the emergency declaration as well as a spending bill aimed at preventing the repetition of the government's closure.
The declaration of an emergency could give him access to billions of dollars. Trump announced the plan after Congress refused to finance the wall.
Within hours, the first legal challenge to the national emergency declaration was launched.
A liberal rights group, Public Citizen, sued on behalf of a nature reserve and three Texas landowners who were informed that the wall could be built on their properties.
How did the Democrats react?
California Governor Gavin Newsom called the president's decision "political theater".
"He was embarrassed and his base needs to be nurtured," he told reporters.
"Fortunately, Donald Trump is not the last word, the courts will be the last word," he added.
New York State Democratic Attorney General Letitia James said that the state would not "defend this abuse of power and fight with all the legal tools at our disposal."
The American Civil Liberties Union (ACLU) said it would take legal action in the coming days to crack down on "this obviously illegal leadership action."
On Friday, the two top Democrats – Speaker of the House of Representatives Nancy Pelosi and Senate Democratic Leader Chuck Schumer – announced that they would contest the "takeover by a disappointed president" in Congress. and the courts.
"The president's illegal declaration on a non-existent crisis is a great violence for our Constitution and makes America less secure, by depriving the necessary defense funds urgently for the security of our armed forces and our country," he said. the press release.
Ms. Pelosi also relied on a remark from Mr. Trump in response to a question from a reporter, in which he stated that he "did not need to do it" .
Analysts suggest that this remark could undermine Trump's case that the country is facing an emergency.
What did Mr. Trump say?
Making the announcement at White House Rose Garden, the president said the emergency would allow him to get nearly $ 8 billion for the wall.
The money should be diverted from military construction projects and existing efforts to combat drug trafficking.
That's still way less than the $ 23 billion cost of the wall along the border.
Mr. Trump agreed to be sued for the move and predicted that the emergency order would result in legal proceedings that would likely result in the Supreme Court.
"We will face the national security crisis on our southern border," he said.
"We have a drug invasion, a gang invasion, an invasion of people, and that's unacceptable.
"Everyone knows that the walls work."
Later, Mick Mulvaney, White House's acting chief of staff, told reporters that Trump's decision "does not create any precedent".
"It's already an authority given to the president, it's not like he did not get what he wanted, he waved a magic wand and took a lot of money," he said. .
By Jon Sopel, Editor-in-Chief of the BBC in North America
The problem with nuclear power is that there are spin-offs. This has been presented as a predictable partisan issue.
On one side of the wall, Republicans; On the other side, the Democrats. But by becoming nuclear, the president made the situation more complicated than that. There are many Republicans – in the Senate and in the House – deeply concerned about what Mr. Trump is doing.
Why? Because the constitutional arrangement of the United States stipulates that Congress – not the president – controls the purse strings and allocates funds.
This is an important land grab by the president. This undermines their position and sets a very dangerous precedent.
Can Congress stop Trump 's emergency move?
The National Emergencies Act contains a clause that allows Congress to end the emergency status if both Houses vote it – and the President does not have the right of veto.
With a comfortable majority in the House, Democrats could pass such a resolution in the Senate. Republicans control the Senate, but a number of Republican senators have expressed their discomfort with the fact that the president is citing a national emergency.
Among the dissident Republicans are the 2012 presidential candidate and new senator of Utah Mitt Romney, Florida senator Marco Rubio, and Maine senator, Susan Collins, who said this decision had a " doubtful constitutionality ".
The resolution, however, would still require Trump's signature, which would allow him to veto it. A supermajority in both houses of Congress is necessary to overturn a presidential veto.
What is a national emergency?
The law on national emergencies is intended for periods of national crisis. Trump said there was a migration crisis at the southern border of the country – a claim strongly refuted by migration experts.
The largest number of illegal immigrants who settle in the United States each year are those who stay in the country after their visa expires.
By declaring a national emergency, the president would have access to special powers to circumvent the usual political process. He would be able to divert money from military budgets or disaster relief to pay for the wall.
Emergency declarations by former presidents have been widely used to deal with foreign policy crises, notably to prevent terrorist-related entities from accessing funds or to prohibit investments in countries. associated with violations of human rights.
- The law on national emergencies explained
Trump's decision to enforce the necessary powers to overcome a partisan stalemate about border security has hit politicians on both sides of the aisle as a deviation from the intended use of the law.
"It would be a pretty dramatic extension of how it was used in the past," Republican Senator Ron Johnson said.