President Trump speaks Sunday on the South Lawn of the White House. (Alex Brandon/AP)

When President Trump said he was considering declaring a “national emergency” allowing him to bypass Congress to build a wall on the southern border, the immediate reaction among many constitutional lawyers was he can’t. Of course he can’t.

But other legal voices weighed in quickly. Not so fast, they said. Maybe he can. And even if he can’t, who’s going to stop him, and how?

Like other unsettled Constitutional arguments, there will be no clear answer until the Supreme Court rules, if it ever does.

Trump has not been explicit about just what he has in mind. Among the most likely scenarios would be to declare the emergency and use it to move funds set aside for one purpose and redirect them to building the wall.

Theoretically, there are ways this could be done legally. There’s a little-known section of the U.S. Code governing the military, for instance, that gives the defense secretary the authority to undertake military construction projects “not otherwise authorized by law” to support any troops deployed in a national emergency requiring the use of the armed forces.

The administration would argue that the section amounts to congressional authorization, meeting one of the crucial tests of constitutionality established by the Supreme Court in 1952 when it struck down — in Youngstown Sheet & Tube Co. vs. SawyerPresident Harry S. Truman’s seizure of the country’s steel mills during the Korean War.

Because the Constitution explicitly says that “no money shall be drawn from the Treasury” without an appropriation by Congress and makes no exception for national emergencies, some experts, such as Harvard Law School’s Noah Feldman in a Bloomberg opinion column, argue that any such maneuver is inherently unconstitutional.

But presidents have been moving money around for years to accomplish aims unauthorized by Congress, using terminology such as “reprogramming” or “contingency” funding to do as they please.

And for decades, Congress has been passing laws allowing presidents to declare national emergencies in matters great and small. Elizabeth Goitein of the Brennan Center for Justice identified 136 emergency powers authorized by Congress, 96 of which can be “activated by the president alone. . . . Some powers seem almost absurdly mundane, such as the one granting Coast Guard officials the ability to serve as notaries public during a national emergency. But others allow the president to shut down or take over radio stations and even suspend a law that prohibits government testing of chemical and biological weapons on unwitting human subjects.”

In 1976, Congress enacted the National Emergencies Act, which authorizes the president to declare emergencies for a variety of purposes — great and small — but offers no definition of emergency.

Any court battle would probably be fought around that definition and its validity, according to Walter Dellinger, head of the Justice Department’s Office of Legal Counsel during the Clinton administration.

That office, which advises the president on constitutional issues, “has an obligation to determine that there is actually a basis” for a declaration of emergency “and to resign if there is not.” It would be “a critical moment for the rule of law” both for them and the attorney general, Dellinger said.

Bobby Chesney, a professor at the University of Texas School of Law, agreed that much would depend on how the Trump administration defines “emergency” and whether the claim is convincing to the courts.

“I think most of us fully understand there is no emergency,” he said. “But it doesn’t follow that everyone will see it that way.”

That could include the Supreme Court, he said, citing its 2018 decision in Trump v. Hawaii, upholding Trump’s travel ban after it had been rejected by multiple lower courts.

Even if Trump’s actions are constitutionally suspect, Chesney said, it could be difficult to stop them. While the Democratic-controlled House might move to block such a use of presidential power, it is unlikely to be joined by the Republican-controlled Senate. If Congress did act, it would need a veto-proof majority, the remotest of possibilities.

Getting into court would pose a challenge as well, Chesney said. Finding someone with standing to sue — meaning a litigant who can show actual harm — would be the first hurdle.

The most “plausible” challenge might come from someone whose land is taken via eminent domain as a result of Trump’s wall construction. But that would be far down the road in the process. Conceivably, Chesney said, a contractor who does not receive money for a defense project because it was diverted to the wall would have standing in court.

But the real obstacle would be the Supreme Court’s traditional deference to presidential authority, particularly when national security is invoked.

“There are any number of district court judges” who would be happy to enjoin Trump’s plans, as they have in dozens of immigration cases, Chesney said. “But it would be harder to get an appeals court” to do the same thing and even more challenging at the Supreme Court.

Deanna Paul contributed to this report.