In disputes over protests, deportations and tariffs, the president has invoked statutes that may not provide him with the authority he claims.

To hear President Trump tell it, the nation is facing a rebellion in Los Angeles, an invasion by a Venezuelan gang and extraordinary foreign threats to its economy.

Citing this series of crises, he has sought to draw on emergency powers that Congress has scattered throughout the United States Code over the centuries, summoning the National Guard to Los Angeles over the objections of California’s governor, sending scores of migrants to El Salvador without the barest hint of due process and upending the global economy with steep tariffs.

Legal scholars say the president’s actions are not authorized by the statutes he has cited and are, instead, animated by a different goal.

“He is declaring utterly bogus emergencies for the sake of trying to expand his power, undermine the Constitution and destroy civil liberties,” said Ilya Somin, a libertarian professor at Antonin Scalia Law School who represents a wine importer and other businesses challenging some of Mr. Trump’s tariffs.

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