But it’s been weakened over the years, often with the collusion of Congress, which enacts flexible spending laws, and by the courts, in their silence.

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President Trump’s declaration of a national emergency to build a wall on the southern border “shines the brightest of lights on how much power Congress has given away,” tweeted Jack Goldsmith, a Harvard Law School professor and former assistant attorney general under President George W. Bush, “and how much extraordinary power presidents have amassed.”

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Trump invoked both coequal branches Friday, citing the National Emergencies Act passed by Congress in 1976 in support of spending more money on a wall than Congress chose to provide. At the same time, he predicted that the question of his power to do so will inevitably rest with the Supreme Court.

A historic decision, then, may be approaching. Despite at least 58 emergency declarations since 1976, the high court has yet to confront the issues raised by them and the laws authorizing them.

Wherever the court lands will make history: If it strikes down Trump’s action or upholds it, the justices will redefine presidential power; if it refuses to consider the merits of the challenges, deciding instead that it is a “political question,” the court could foreclose challenges against future presidents who go even further.

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The long-term importance of this case deeply worries even some conservatives who believe Trump is acting constitutionally.

“Frustrated with Washington, President Trump believes he has no choice but to take this action today,” Kay Coles James, president of the Heritage Foundation, said in a statement Friday. “While it is strictly constitutional . . . this creates a dangerous precedent for future administrations.”

In 2011, the Heritage Foundation’s “guidance for lawmakers” told them flatly that their power of the purse means presidents “can’t spend what you don’t approve.”

With promises of lawsuits Friday from the state of California, the American Civil Liberties Union, the leadership of the House and many others, clashes are imminent. If the courts say they don’t have standing to sue, which is possible, next in line would be landowners of property the government tries to take for the wall.

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Litigants will muster at least several lines of attack, lawyers said Friday.

They will challenge the constitutionality of Trump’s actions, citing Madison’s “weapon.” “The power of the purse is the most important checking and balancing tool that the Congress holds with respect to the separation of powers,” said Peter Shane, a law professor at Ohio State University.

The framework for a constitutional challenge will be the Supreme Court landmark ruling on presidential power, the 1952 decision in Youngstown Sheet & Tube Co. v. Sawyer, which struck down President Harry S. Truman’s seizure of American steel plants. “Youngstown,” as lawyers call it, will be “the most famous Supreme Court case in Washington for the next couple of months,” said Rep. Jamie B. Raskin (D-Md.)

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Under the Youngstown test, a president who exercises power “incompatible with the expressed or implied will of Congress” is in trouble. His power is at “the lowest ebb,” Justice Robert Jackson wrote in a legendary concurrence in the case. With Trump repeatedly saying the emergency arose because Congress would not pay for his wall, he may be in the ebbing zone.

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But the federal courts try to avoid constitutional issues when they can.

So another line of attack will be the legitimacy of the emergency Trump declared. As a chorus of legal experts tweeted Friday, Trump didn’t help his case by declaring in the Rose Garden at the White House that “I didn’t need to do this . . . I just want to do it faster.”

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“Whatever a national emergency may be, that’s not it,” tweeted former acting U.S. solicitor general Neal Katyal. “That quote is going right in the lawsuit.”

Finally, litigants will challenge Trump’s use of the specific laws he said he is using to come up with the money.

Among them, for example, is a law granting the president “emergency military construction authority.” It applies to a situation “that requires the use of the armed forces,” permitting the Pentagon to “undertake military construction projects . . . not otherwise authorized by law that are necessary to support such use of the armed forces.”

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“There is no emergency that justifies the use of the military,” said Sam Berger, a former senior lawyer at the Office of Management and Budget and the vice president of the liberal-leaning Center for American Progress.

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And even if there was, he said, the construction would have to be necessary to support the military.

In the end, if Trump loses, it may be a deeply conservative principle that does it: the sanctity of private property.

Much of the land he would need to build the wall is in private hands, and “the president can’t take private property to build something” without the permission of Congress, said Robert Turner, a conservative lawyer and member of the Federalist Society. The president, he believes, is on shaky legal ground.

“My sense is this is going to be a hard fight for the president,” he said. “I can’t say I’m certain he’s going to lose. But it would surprise me if he wins.”