President Donald Trump declared a national emergency along the southern border and predicted his administration would end up defending it all the way to the Supreme Court.
That might have been the only thing Mr Trump said on Saturday that produced near-universal agreement.
The American Civil Liberties Union announced its intention to sue less than an hour after the White House released the text of Mr Trump's declaration that the "current situation at the southern border presents a border security and humanitarian crisis that threatens core national security interests and constitutes a national emergency".
Nonprofit watchdog group Public Citizen filed a suit later, urging the United States District Court for the District of Columbia to "bar Trump and the US Department of Defence from using the declaration and funds appropriated for other purposes to build a border wall".
House Speaker Nancy Pelosi and several Democratic state attorneys general already have said they might go to court.
The coming legal fight seems likely to hinge on two main issues: Can the president declare a national emergency to build a border wall in the face of Congress' refusal to give him all the money he wanted and, under the federal law Mr Trump invoked in his declaration, can the Defence Department take money from some congressionally approved military construction projects to pay for wall construction?
The Pentagon has so far not said which projects might be affected.
But after weeks of publicly ruminating whether to act, Mr Trump's signature on the declaration set in motion a quick march to the courthouse.
Mr Trump relied on the National Emergencies Act of 1976, which Congress adopted as a way to put some limits on presidential use of national emergencies. The act requires a president to notify Congress publicly of the national emergency and to report every six months. The law also says the president must renew the emergency every year, simply by notifying Congress. The House and Senate also can revoke a declaration by majority vote, though it would take a two-thirds vote by each house to override an expected presidential veto.
Beyond that, though, the law doesn't say what constitutes a national emergency or impose any other limits on the president.
The broad grant of discretion to the president could make it hard to persuade courts to rule that Mr Trump exceeded his authority in declaring a border emergency. "He's the one who gets to make the call. We can't second-guess it," said John Eastman, a professor of constitutional law at the Chapman University School of Law.
But other legal experts said the facts are powerfully arrayed against the president. They include government statistics showing a decades-long decline in illegal border crossings as well as Mr Trump's rejection of a deal last year that would have provided more than the nearly $US1.4 billion ($NZ2.03b) he got for border security in the budget agreement he signed on Friday. Opponents of the declaration also are certain to use Mr Trump's own words at his Rose Garden news conference to argue that there is no emergency on the border.
"I could do the wall over a longer period of time," Mr Trump said. "I didn't need to do this, but I'd rather do it much faster."
Republican Justin Amash of Michigan said Congress made a conscious choice not to give Mr Trump what he wanted. "A prerequisite for declaring an emergency is that the situation requires immediate action and Congress does not have an opportunity to act," Mr Amash said on Twitter.
A national emergency declaration for a non-emergency is void. A prerequisite for declaring an emergency is that the situation requires immediate action and Congress does not have an opportunity to act. @POTUS @realDonaldTrump is attempting to circumvent our constitutional system.— Justin Amash (@justinamash) February 15, 2019
ACLU executive director Anthony Romero said Mr Trump's remarks are an admission that there is no national emergency. "He just grew impatient and frustrated with Congress," Mr Romero said in a statement that also said the rights group would file a lawsuit next week.
Trying to turn the president's words against him failed in the challenge to Mr Trump's ban on travel to the United States by citizens of several mostly Muslim countries. The ban's opponents argued that Mr Trump's comments as a candidate and as president showed the ban was motivated by anti-Muslim bias, not concern about national security. Lower courts struck down the ban, but the Supreme Court upheld it in a 5-4 vote last year.
Mr Trump said he expected to lose in lower courts that he claims have been unfair to him, particularly if lawsuits are filed in California. "Hopefully, we'll get a fair shake and we'll win in the Supreme Court, just like the ban," he said.
Beyond the challenge to Mr Trump's authority to declare an emergency, lawsuits also are expected to focus on the military construction project law that allows the re-allocation of money in a national emergency.
Professor Eastman said he doubts that the Supreme Court would try to interfere with Mr Trump's decision to send the military to the border and then authorize the use of money from other Defence Department construction projects to build miles of a border wall. "The president is authorised to make those judgments, not some judge in San Francisco," he said.
But the ACLU's suit will argue that Congress allowed for flexibility in using money it appropriated for projects needed to support the emergency use of the military forces, like overseas military airfields in wartime.
Several legal experts said claims that the building of the wall is not the kind of project contemplated in the military construction law could be more difficult to rebut because border security is more like a law enforcement issue than a military emergency.
But Professor Shane from Ohio State said, "It's hard to know how exactly this is going to unfold politically or judicially."