A coalition of 16 US states led by California is suing President Donald Trump's administration over his decision to declare an emergency to raise funds for a Mexican border wall.
Mr Trump made the declaration on Friday to bypass Congress after it refused to approve $5.7bn (£4.4bn) for the wall.
The states say they want to block Mr Trump's "misuse of presidential power".
The Democrats oppose funding Mr Trump's barrier, a key campaign pledge, and have vowed to contest his declaration.
The president's announcement came after he signed a spending bill to avoid another government shutdown that granted him only $1.375bn for new border barriers.
Mr Trump said he did not need to declare the emergency but did so in the hope of obtaining the funds for the wall more quickly. Analysts say these comments could undermine his legal arguments.
What does the lawsuit say?
The lawsuit filed on Monday seeks a preliminary injunction that would stop Mr Trump acting on his emergency declaration while a legal battle takes place in the courts, the Washington Post reports.
"We're suing President Trump to stop him from unilaterally robbing taxpayer funds lawfully set aside by Congress for the people of our states. For most of us, the office of the presidency is not a place for theatre," California Attorney General Xavier Becerra said.
Joining California in the lawsuit were Colorado, Connecticut, Delaware, Hawaii, Illinois, Maine, Maryland, Minnesota, Nevada, New Jersey, New Mexico, New York, Oregon, Virginia and Michigan.
All states but Maryland are governed by Democrats. They argue that President Trump's order to divert funds to pay for the wall would cost them millions of dollars, damaging their economies.
The White House has not commented.
California Governor Gavin Newsom dismissed the president's decision as "political theatre" while New York state's Democratic Attorney General Letitia James promised to "fight back with every legal tool at our disposal".
The lawsuit was filed in the court for the Northern District of California.
How did Mr Trump declare the emergency?
The president said the emergency would allow him to get almost $8bn for the wall, still considerably short of the estimated $23bn cost of the barrier along almost 2,000 miles (3,200km) of border.
Mr Trump accepted that he would be sued for the move, and predicted that the case was likely to end up in the Supreme Court.
"We're going to confront the national security crisis on our southern border," he said. "Everyone knows that walls work."
Critics, ,however accused the government of a "manufactured crisis", saying the president's assertions about the situation were untrue.
Following the declaration on Friday, a liberal advocacy group, Public Citizen, sued on behalf of a nature reserve and three Texas landowners who were told the wall could be constructed on their properties.
What is a national emergency?
The National Emergencies Act is intended for times of national crisis. Mr Trump has claimed that there is a migration crisis at the nation's southern border - a claim strongly refuted by migration experts.
The largest number of illegal migrants settling in the US each year is those who stay in the country after their visas expire.
Declaring a national emergency would give the president access to special powers that effectively allow him to bypass the usual political process, and he would be able to divert money from existing military or disaster relief budgets to pay for the wall.