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.

Crisis is Mr. Trump’s brand. When he took office the first time, he promised to end “American carnage.” When he announced his most recent re-election campaign, he said he would reverse “staggering American decline.” Ever since he first ran for president in 2015, he has argued that only he can restore the country to greatness.

Now in office again, he is converting that rhetoric into policy. Mr. Trump says that events and circumstances largely considered routine amount to emergencies that allow him to invoke powers rarely sought by his predecessors but embedded in statutes by lawmakers who wanted to ensure presidents could act quickly and aggressively to confront authentic crises.