WASHINGTON (NYTIMES) - A coalition of 16 states, including California and New York, on Monday (Feb 18) challenged US President Donald Trump in court over his plan to use emergency powers to spend billions of dollars on his border wall.

The lawsuit is part of a constitutional confrontation that Mr Trump set off on Friday (Feb 15) when he declared that he would spend billions of dollars more on border barriers than Congress had granted him. The clash raises questions over congressional control of spending, the scope of emergency powers granted to the president, and how far the courts are willing to go to settle such a dispute.

The suit, filed in Federal District Court in San Francisco, argues that the president does not have the power to divert funds for constructing a wall along the Mexican border because it is Congress that controls spending.

Mr Xavier Becerra, the attorney general of California, said in an interview that the president himself had undercut his argument that there was an emergency on the border.

"Probably the best evidence is the president's own words," he said, referring to Mr Trump's speech on Feb 15 announcing his plan.

"I didn't need to do this, but I'd rather do it much faster," Mr Trump had said.

The lawsuit, California et al. v. Trump et al., says that the plaintiff states are going to court to protect their residents, natural resources and economic interests.

"Contrary to the will of Congress, the president has used the pretext of a manufactured 'crisis' of unlawful immigration to declare a national emergency and redirect federal dollars appropriated for drug interdiction, military construction and law enforcement initiatives towards building a wall on the United States-Mexico border," the lawsuit says.

Congress is on its own separate track to challenge the president's declaration. The Democrat-controlled House of Representatives may take a two-prong approach when it returns from a recess. One would be to bring a lawsuit of its own.

Lawmakers could also vote to override the declaration that an emergency exists, but it is doubtful that Congress has the votes to override Mr Trump's certain veto, leaving the courts a more likely venue.

The dispute stems from steps Mr Trump said he would take after lawmakers granted him only US$1.375 billion (S$1.87 billion) for new border barriers, legislation he signed last week to avoid another government shutdown.

Mr Trump asserted the power to tap three additional pots of money on his own: US$600 million from a Treasury Department asset forfeiture fund for law enforcement priorities; about US$2.5 billion from a military antidrug account, most of which would be siphoned from other military programmes the Pentagon largely has yet to identify; and US$3.6 billion in military construction funds he said he could redirect by invoking an emergency-powers statute.

Presidents have invoked emergency-powers statutes nearly five dozen times since Congress enacted the National Emergencies Act of 1976, but never before has one been used to make an end-run around Congress after it rejected funding for a particular policy.

But as the debate over Mr Trump's action shifts to courtrooms, legal experts warned that its fate may turn less on such high constitutional principle and more on complex legal issues - from whether plaintiffs can establish that the case is properly before the courts, to how to interpret several statutes.

"Even though Trump's political manoeuvre to get around an uncooperative Congress looks like it stretches the Constitution, the questions presented in court will raise ordinary and complicated issues of administrative law," said Prof Peter M. Shane, an Ohio State University law professor and co-author of a separation-of-powers casebook.

Two cases had already been filed after Mr Trump's announcement on Feb 15 - one by the nonprofit watchdog group Public Citizen, representing several Texas landowners and a Texas environmental group, and the other a case jointly brought by the Centre for Biological Diversity, Defenders of Wildlife and the Animal Legal Defence Fund.

At least two other lawsuits are expected to be filed later this week. The American Civil Liberties Union has announced its intention to file a case, but has not yet publicly identified its client. The other case will be brought by Protect Democracy, another watchdog group, and the Niskanen Centre, a centre-right policy institute, on behalf of El Paso County and the Border Network for Human Rights.

Many critics have challenged whether an emergency truly exists on the Southern border that a wall would solve, pointing to government data showing that the number of people crossing illegally has dropped significantly over the past generation and that most drugs are smuggled through ports of entry.

The president has argued, without proof, that the emergency declaration is warranted because the migrants "invading" the United States across the Mexico border have caused epidemics of crime and drug use.

Legal specialists expected the Justice Department to urge a court not to consider facts about the border or Mr Trump's words, but rather to defer to the president's decision. The courts have a long history of being reluctant to substitute their own judgment for the president's about a security threat.

The Trump administration will have a powerful argument to invoke: In the National Emergencies Act, Congress defined no standard for what conditions have to be met before a president may determine that a qualifying crisis exists.

But before a judge could weigh whether Mr Trump invoked the statute legitimately, he or she would have to decide whether the dispute is properly before the court in the first place.

Plaintiffs will need to establish standing by showing that they are suffering some particular injury from what Mr Trump is doing. Several of the lawsuits involve people who own land or represent communities along the Mexican border in Texas, where Mr Trump has put the focus of his emphasis on the need for more barriers.

But it is not clear whether any of the fencing will be built in California or New Mexico, two of the states in the lawsuit, and it certainly will not be built in other states involved in the litigation, like New York, New Jersey, Washington or Hawaii.

Mr Becerra, California's attorney general, suggested that plaintiffs in the states' lawsuits have standing for reasons that are unrelated to whether any portions of Mr Trump's wall will be built in their territory, arguing that "the president's unconstitutional action could cause harms in many parts of the country."

People in California and other plaintiff states could "lose funding that they paid for with their tax dollars, money that was destined for drug interdiction or for the Department of Defence for military men and women and military installations," he said in the interview.

Further complicating matters, the administration has said it intends to spend the funds in sequence, starting with the US$1.375 billion Congress appropriated, and reaching the emergency-power military-construction fund last. The Justice Department is likely to argue that if no disputed spending is imminent, the case is not ripe for litigation and should be dismissed.

Mr Ian Bassin, the executive director of Protect Democracy, said that El Paso County would probably argue that its economy was being harmed by Mr Trump's emergency declaration because it signalled to businesses and potential tourists that they should stay away.

The Justice Department declined to comment on the wave of lawsuits. Mr Trump has said he expected to be sued and to lose in lower courts, but he predicted he would eventually prevail before the Supreme Court.

But plaintiffs can also challenge whether the administration is interpreting several statutes correctly.

The provision that gives the secretary of defence authority to transfer some Pentagon money from the antidrug account Mr Trump is planning to tap, for example, says its authority may be used "in no case where the item for which funds are requested has been denied by the Congress" - raising the question of whether extra funding for border barriers counts as such a forbidden item.

And the emergency-powers statute Mr Trump plans to use permits military construction spending in an emergency that requires the use of the armed forces for projects "to support such use". That has been used before to build up foreign military bases in a war effort, but litigants challenge whether a permanent wall to help civilian agencies police the border qualifies under that wording.

"Judges will certainly be aware of the larger institutional context when they address those technical issues, but that awareness will not, by itself, determine how the legal questions get resolved," Mr Shane said.