WASHINGTON — As a presidential candidate, Donald J. Trump vowed to refill the cells of the Guantánamo Bay prison and said American terrorism suspects should be sent there for military prosecution. He called for targeting mosques for surveillance, escalating airstrikes aimed at terrorists and taking out their civilian family members, and bringing back waterboarding and a “hell of a lot worse” — not only because “torture works,” but because even “if it doesn’t work, they deserve it anyway.”

It is hard to know how much of this stark vision for throwing off constraints on the exercise of national security power was merely tough campaign talk. But if the Trump administration follows through on such ideas, it will find some assistance in a surprising source: President Obama’s have-it-both-ways approach to curbing what he saw as overreaching in the war on terrorism.

Over and over, Mr. Obama has imposed limits on his use of such powers but has not closed the door on them — a flexible approach premised on the idea that he and his successors could be trusted to use them prudently. Mr. Trump can now sweep away those limits and open the throttle on policies that Mr. Obama endorsed as lawful and legitimate for sparing use, like targeted killings in drone strikes and the use of indefinite detention and military tribunals for terrorism suspects.

And even in areas where Mr. Obama tried to terminate policies from the George W. Bush era — like torture and the detention of Americans and other people arrested on domestic soil as “enemy combatants” — his administration fought in court to prevent any ruling that the defunct practices had been illegal. The absence of a definitive repudiation could make it easier for Trump administration lawyers to revive the policies by invoking the same sweeping theories of executive power that were the basis for them in the Bush years.