By Charlie Savage, New York Times

WASHINGTON — As a presidential candidate, Donald Trump vowed to refill the cells of the Guantánamo Bay prison and said U.S. 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 Barack Obama’s have-it-both-ways approach to curbing what he saw as overreaching in the war on terrorism.

Over and over, 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. Trump can now sweep away those limits and open the throttle on policies that 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 Obama tried to terminate policies from the George W. Bush era — such as 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.

Two decisions by Obama in 2009 set the tone for his leave-it-on-the-table approach. They involved whether to keep indefinite wartime detentions without trial and to continue using military commission prosecutions — if not at the Guantánamo prison, which he had resolved to close, then at a replacement wartime prison.

Told that several dozen detainees could not be tried for any crime but would be particularly risky to release, and that a handful might be prosecutable only under the looser rules governing evidence in a military commission, Obama decided that the responsible policy was to keep both the tribunals and the indefinite detentions available.

The president refused to use either power on newly captured terrorism suspects, instead prosecuting them in civilian court. But by leaving the options open, he helped normalize them and left them on a firmer legal basis.

Obama followed a similar course with several national security practices that became controversial during his first term. After his use of drones to kill terrorism suspects away from war zones led to mounting concerns over civilian casualties and other matters, he issued a “presidential policy guidance” in May 2013 that set stricter limits. They included a requirement that the target pose a threat to Americans — not just to U.S. interests — and that there would be near certainty of no bystander deaths.

But the Obama administration also successfully fought in court to establish that judges would not review the legality of such killing operations, even if a U.S. citizen was the target. Trump — who has said he would “bomb the hell out of ISIS,” beyond what Obama is doing, and go after civilian relatives of terrorists, prevailing over any military commanders who balked — could scrap the internal limits while invoking those precedents to shield his acts from judicial review.

Similarly, after a surge of criminal prosecutions against people who leaked secret information to the news media and bipartisan outrage at aggressive investigative tactics targeting journalists, the Obama Justice Department issued new guidelines for leak investigations intended to make it harder for investigators to subpoena reporters’ testimony or phone records. It also decided not to force a reporter for The New York Times to testify in a leak trial or face prison for contempt.

But the Obama administration also successfully fought in court to establish that the First Amendment offers no protection to journalists whom the executive branch chooses to subpoena to testify against confidential sources. Trump, who has proposed changing libel laws to make it easier to sue news organizations, could abandon the Obama-era internal restraints and invoke the Obama-era court precedent to adopt more aggressive policies in leak investigations.

Geoffrey R. Stone, a University of Chicago law professor who is a friend and adviser to Obama, defended the president’s approach. He said that after 2010, when Republicans took over the House, internal executive branch restraints were the only option because Congress was not going to enact legislation limiting national security powers.

He also said that even if Obama had gotten rid of indefinite detention or military tribunals, Trump could have brought them back.

“Short of legislation that restricts things, there is not much a president could do in these matters to restrain a successor,” Stone said.

Still, Bruce Ackerman, a Yale University law professor who is helping with a lawsuit that claims that Obama is waging an illegal war against the Islamic State because Congress never specifically authorized it, said Obama had contributed to the growth of executive powers that Trump would inherit. That includes “the fundamental institutional legacy” of relying on executive branch lawyers to produce creative legal opinions clearing the way for preferred policies, Ackerman said.

The two areas where Obama broke most cleanly with Bush-era practices were torture and the indefinite military detention of Americans and other terrorism suspects arrested on domestic soil. Obama issued an executive order requiring interrogators to use only techniques approved in the Army Field Manual, and he later signed a bill codifying that rule into statute. He also resisted repeated calls by Republicans to put newly captured terrorism suspects arrested in the United States into Guantánamo-style military detention.

But the Obama administration also ruled out criminal investigations into Bush-era officials for involvement in torture practices that the Justice Department had blessed as legal under a sweeping theory that the commander in chief could not be bound by anti-torture laws.

And the Obama administration fought lawsuits brought by Jose Padilla, a U.S. terrorism suspect who had been imprisoned and interrogated as an “enemy combatant.” The administration successfully argued that courts should dismiss the litigation without ruling on whether his treatment had been lawful, preventing any clear repudiation of the Bush-era legal theory.

A spokesman for Obama’s National Security Council declined to comment. But Gregory B. Craig, who was Obama’s first White House counsel and participated in early policy deliberations about what to do about Guantánamo-style policies, said that in 2009, the president “was not thinking about 10 years out, but about 10 days out.” And he especially did not want to send signals to Republicans that he was a zealot or out for revenge, Craig said.

Obama, Craig said, “was thinking about working with Republicans and developing postpartisan relations on Guantánamo-related national security issues, not about what was going to happen a decade later.”

Anthony D. Romero, the executive director of the American Civil Liberties Union, which has sharply criticized the Obama administration’s approach, said it was now clear that Obama had “missed an opportunity” to fundamentally reject the sort of policies that the Bush administration put in place after the Sept. 11, 2001, attacks.

“Obama’s failure to rein in George Bush’s national security policies hands Donald Trump a fully loaded weapon,” Romero said. “The president’s failure to understand that these powers could not be entrusted in the hands of any president, not even his, have now put us in a position where they are in the hands of Donald Trump.”

To Learn More:

Obama Justice Dept. Releases George W. Bush’s Legal Justification for Warrantless Surveillance with Key Sections Still Censored (by Steve Straehley and David Wallechinsky, AllGov)

Obama Refuses to Release Bush’s Legal Excuse for Illegal Surveillance (by Noel Brinkerhoff, AllGov)

Obama Clashes with Journalists over Confidentiality Law (by Noel Brinkerhoff, AllGov)

Obama: Torture Okay if Just Following Orders (by Noel Brinkerhoff and David Wallechinsky, AllGov)

Obama Administration Again Defends Bush Wiretapping (by Noel Brinkerhoff, AllGov)

Will Obama Release Secret Bush Torture Memos? (AllGov)