The legal battle over President Trump’s declaration of a national emergency to pay for a border wall grew Tuesday with another lawsuit challenging his actions, a fight that could explore sweeping constitutional questions as judges decide how much deference to give presidential power and what — if any — evidence Trump needs to support his claims of a dire situation needing a solution that sidesteps Congress.

The lawsuits challenging Trump’s order argue that there is no evidence of an emergency at the southern border, assail his action as unconstitutional and accuse him of manufacturing a crisis. Trump said that only a border wall could prevent “an invasion of our country with drugs, with human traffickers, with all types of criminals and gangs.”

But the National Emergencies Act, which Trump cited in his declaration, does not specifically say what constitutes an emergency, said Stephen I. Vladeck, a University of Texas law professor and constitutional law expert.

And while courts tend to defer to government officials who invoke national security issues, the fight over Trump’s emergency decree — which the president said was needed to fund his long-promised wall — marks a new frontier for an administration that has been willing to take action and let the courts sort it out.

“We’ve really never had litigation under the National Emergencies Act before, so to some degree we’re in uncharted territory,” Vladeck said.

Vladeck said that rather than seeking specific evidence of an emergency or trying to second-guess whether the border situation is an emergency, he expects the courts to look at “the meaning of hitherto obscure statutes.” Such an examination could hinge on whether Trump has authority to carry out narrow, specific actions, such as the president’s move to divert $3.6 billion in previously appropriated military construction funds for the construction, experts said.

The American Civil Liberties Union on Tuesday joined the fray, filing a lawsuit on behalf of the Sierra Club and the Southern Border Communities Coalition against the emergency declaration. That came a day after a coalition of 16 states filed suit.

While Trump has characterized the situation at the border as a crisis, advocates have described a different struggle there, with many of the people seeking entry to the United States arriving essentially as refugees, fleeing violence and hunger in Central America. Some are being forced to wait in dangerous places in Mexico while their asylum claims are processed.

“We know firsthand there is no national emergency at the border, only the one that exists in Trump’s mind,” Vicki Gaubeca, director of the Southern Border Communities Coalition, told reporters Tuesday. “The southern border is a place of hope and opportunity, not a place of hate. . . . Trump is manufacturing a crisis in one of the safest, most vibrant places in our nation to fulfill a campaign promise based on ignorance, hate and fear.”

Dror Ladin, a staff lawyer with the ACLU’s National Security Project, described the legal challenge to the emergency as needed because Trump is ignoring Congress’s decisions about how to spend taxpayer money.

“The founders were very worried about letting an executive freely spend on whatever projects he decided in his own judgment were important, when the will of the people were against it,” Ladin said.

John Yoo, a law professor at the University of California at Berkeley and a Justice Department lawyer during the administration of President George W. Bush, said that while he’s “not a big fan of Trump or his immigration policies,” he thinks “the Supreme Court will uphold the emergency declaration.”

Yoo said he believes that the administration will cite “the only relevant” Supreme Court precedent, a challenge in 1981 to President Jimmy Carter’s use of emergency power to block Iranian assets in the wake of the Iran hostage crisis. In an 8-to-1 ruling, the court upheld Carter’s action.

The court “never asked if the Iranian hostage crisis was a real emergency,” said Yoo, who during his time in government wrote memos asserting dramatic White House power. “You can’t define an emergency beforehand. That’s why it’s an emergency. The moment you say, ‘An emergency has these three or four characteristics,’ you’re defeating the purpose” of the laws authorizing emergency actions.

Vladeck said the litigation playing out in court will ultimately be “a lot more specific and I think a lot more nuanced” than many initial responses.

“It’s not surprising that the president’s critics are portraying his actions in that manner, but courts for a whole host of reasons are going to, where possible, decide these cases on more technical grounds,” he said. “I think it’s quite possible that we’re heading toward a rash of lawsuits that have at their core the perceived illegitimacy of this emergency but that actually turn on very different questions about the appropriate uses of repurposed military construction funds.”

Trump knew beforehand that his emergency declaration would face legal hurdles, and during his announcement of the declaration Friday in the Rose Garden, he laid out his view of how the lawsuits might unfold.

“We will have a national emergency, and we will then be sued, and they will sue us in the 9th Circuit, even though it shouldn’t be there, and we will possibly get a bad ruling, and then we’ll get another bad ruling, and then we’ll end up in the Supreme Court, and hopefully we’ll get a fair shake and we’ll win in the Supreme Court,” Trump said.

The ACLU lawsuit, like the complaint filed by the coalition of states, was entered in U.S. District Court for the Northern District of California; the group said this was because the Sierra Club’s headquarters are located in Oakland.

In their lawsuit, the coalition of states — most of which are not located along the southern border — included explanations of their respective standing for challenging the declaration in court. Some said it could strip them of funding for National Guard troops, drug interdiction and military construction projects.

Other opponents of Trump’s declaration that did not join the lawsuit are trying to determine if they can do so. Massachusetts Attorney General Maura Healey called Trump’s action an “illegal power grab” but said her staff is researching whether it would directly affect their state. Washington state is taking a similar approach.

Other states on the border showed no signs of joining up. In a statement, Arizona Attorney General Mark Brnovich, a Republican, defended Trump’s “broad authority to declare a national emergency with existing powers” Congress has given him.

“Congress has failed to act on the long-standing crisis at our southern border, and the president should be doing everything he legally can to improve border security,” he said.

The outcome of the challenges to Trump’s order could ultimately depend on how much deference courts are willing to give him as a president arguing there is a national security issue, said Bobby Chesney, a constitutional law professor at the University of Texas.

None of the laws Trump has invoked have specific definitions for emergencies, Chesney said.

“In any normal situation, that would be the end of the matter,” he said, meaning that, absent a clear definition, the president would be given wide latitude to come up with his own definition of an emergency.

But Trump’s comments could complicate matters, because he “keeps saying things that suggest there’s not an emergency in any layman’s sense of the term,” Chesney said.

During his announcement Friday, Trump described the declaration as needed for speed rather than urgency, telling reporters: “I could do the wall over a longer period of time. I didn’t need to do this, but I’d rather do it much faster.”

Those comments appeared in the lawsuit filed Monday by California, New York and 14 other states assailing Trump’s order as an “unconstitutional and unlawful scheme” that they said would lead to environmental damage in some states and economic damage in others.

California Attorney General Xavier Becerra said Tuesday that Trump’s actions already have undermined his case.

“The president himself has admitted that this is not a national emergency, that there is no real crisis,” Becerra, a Democrat, said in an interview Tuesday. “He himself said he didn’t need to do this declaration.”

Stephen Miller, a senior White House policy adviser, sought to reframe Trump’s remarks about the timetable in an interview over the weekend.

“What the president was saying is that like past presidents, he could choose to ignore this crisis, choose to ignore this emergency as others have,” Miller said on “Fox News Sunday.” “But that’s not what he’s going to do.”

Trump’s claims about the situation at the border are at odds with the facts on the ground, something he dismissed last week by belittling government statistics. Advocates for migrants have argued that the suggestion of an emergency at the border is focusing on the wrong front.

“It’s a historically low number of people crossing,” said Erika Andiola, chief of advocacy for Raices, a Texas nonprofit that is aiding immigrants. “They’re knocking at the door, and there’s not enough help or support from the U.S. government to actually process them.”

In their lawsuit, states say the border is more difficult to cross than ever. Apprehensions are near 45-year lows, down from more than 1.6 million arrests in fiscal 2000 to slightly fewer than 400,000 last year; the number of agents patrolling the U.S.-Mexico border nearly doubled during that period.