After President Trump left the White House in 2021, critics of his norm-breaking use of executive power implored Congress to tighten legal limits on when presidents can unilaterally reshape American government with the stroke of a pen. But lawmakers largely did not act.

On Monday, as Mr. Trump took the oath of office to begin his second term, he asserted a muscular vision of presidential power. He not only revived some of the same expansive understandings of executive authority that were left unaddressed, but went even further with new claims of sweeping and inherent constitutional clout.

Among a blizzard of executive orders, Mr. Trump instructed prosecutors not to enforce a law that bans the popular social media app TikTok until its Chinese owner sells it. President Joseph R. Biden Jr. had signed the measure into law after it passed with broad bipartisan support, and the Supreme Court unanimously upheld it.

Whatever the law’s merits, the Constitution says presidents “shall take care that the laws be faithfully executed.” Mr. Trump offered no clear explanation for how he has any legitimate power to instead suspend the law, making only a vague gesture toward his “constitutional responsibility” for national security, foreign policy “and other vital executive functions.”

Unilateral actions like emergency declarations and executive orders cannot create new legal powers for a president. Instead, they are a vehicle by which presidents exercise legal authority they already have, either because the Constitution has bestowed it upon their office or because Congress passed a law creating it.

People lined up outside the Supreme Court earlier this month to hear arguments in the TikTok case. Mr. Trump has instructed prosecutors not to enforce a law that bans the app even though the justices unanimously upheld it.Credit…Caroline Gutman for The New York Times

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