WASHINGTON » For President Barack Obama, this has been a week to guard his power. He went to court to defend his executive action on immigration while fighting an effort by lawmakers to reverse it. And he vetoed legislation that would have stripped him of his authority to decide the fate of the much-debated Keystone XL pipeline.

Yet this is also a week when Obama is seeking to circumscribe his own power as lawmakers take up his request for retroactive endorsement of his war against Islamic State extremists in Iraq and Syria. Obama proposed that he not be allowed to send large-scale ground forces and that the authorization expire in three years, although he left room to get around that if need be.

The disconnect between muscular assertion of power one minute and voluntary sacrifice of it the next is mirrored on the other side of the political spectrum. Republicans have accused Obama of presiding over a new imperial presidency by exercising his executive powers on immigration, health care and other matters. At the same time, they have complained that he wants to limit his power when it comes to making war against the nation’s enemies.

The curious nature of the debate underscores the changing political environment in Washington. In modern times, presidents of both parties have traditionally taken an expansive view of their own abilities across the board, not just in one area, much as Congress has often tried to reassert itself. But today the political dynamics are not so simple.

"In these highly polarized times, your willingness to say a president has certain powers is intertwined with your trust that the president will exercise those powers wisely," said William A. Galston, who was a domestic policy aide to President Bill Clinton. "As that trust has all but broken down, interpretations of constitutional authority will bump up and down depending on the occupant of the White House."

The debate about the scope of presidential power goes back to the beginning of the republic, when the country’s founders threw out the Articles of Confederation to write a Constitution with an executive to balance out Congress. As the nation grew, the reach of government expanded and the world became more complicated, the balance shifted more and more to the executive. After Watergate, Congress reasserted itself, but presidents have been clawing their way back ever since.

The struggle became more acute under President George W. Bush, who, advised by Vice President Dick Cheney, embraced an assertive interpretation of the commander in chief’s power to fight terrorism by ordering secret surveillance programs that went beyond a 1978 domestic-spying law and detention practices that went beyond the Geneva Conventions. Ultimately, the Supreme Court intervened and determined that in some cases he had gone too far, and Congress pushed back to a degree.

Since taking office, Obama has kept much of the counterterrorism strategy he inherited from Bush, including the surveillance programs. And he has made his own assertions of executive power to back up security decisions, including the authority to kill U.S. citizens, if capture is not feasible, who are deemed to be part of a terrorist network threatening the United States.

But domestic policy is the area in which Obama has clashed most frequently with Congress. Republican critics have complained that he has abused his authority in the way he has enacted his health care program and by deciding to spare up to 5 million unauthorized immigrants from deportation and make them eligible for work permits.

The Supreme Court ruled at one point that Obama had gone too far in asserting his power to fill vacancies during congressional recesses, and a Texas court has now temporarily halted his new immigration policy while considering a lawsuit challenging his power to unilaterally make such a dramatic change.

Obama, conversely, argued that it was Congress that went too far with the Keystone legislation because it would have rewritten the president’s power to decide on pipelines that cross international borders. In his veto message Tuesday, he wrote that the bill tried to "circumvent longstanding and proven processes" and "conflicts with established executive branch procedures."

These clashes are flaring even as Obama seeks to enact limits on the president’s war-making powers in a way that few if any of his predecessors have sought to do. He has already been launching airstrikes against the Islamic State for six months, citing existing authorizations of force passed under Bush, but Obama agreed with critics that Congress should have a role.

The draft measure he sent to Capitol Hill would authorize what he has already been doing in Iraq and Syria while repealing one of those existing authorizations — the one passed in 2002 that paved the way for the invasion of Iraq. But Obama’s proposal would ostensibly bar him and the next president from launching "enduring offensive ground combat operations" against the Islamic State and would expire in three years, requiring his successor to go back to Congress if he or she determined that operations were still necessary.

"This president has the most eclectic approach to separation of powers that is essentially driven by politics," said David B. Rivkin, a lawyer who served in the administrations of Ronald Reagan and the first George Bush and who has worked on constitutional challenges to Obama’s actions.

He stretches his authority on domestic issues "by rewriting statutes," Rivkin said, "while surrendering to Congress his core authority with this" authorization-of-force proposal. "The only unifying factor is political expediency."

The White House, unsurprisingly, has accused the other side of the same thing.

"The Republicans’ protests over the president’s use of executive authority are more about election nullification than constitutional interpretation," said Dan Pfeiffer, Obama’s departing senior adviser. "The Republicans were strong advocates for an expansive view of executive authority when it was their executive in the Oval Office."

Galston, the former Clinton aide, who is now a scholar at the Brookings Institution, said he had been concerned that recent interpretations of a president’s commander-in-chief power under Article II of the Constitution had been too robust. But he noted that it is not inconsistent for Republicans to believe that a president’s role should be more expansive when it comes to defense of the country than it is when it comes to domestic policy, where Congress should be a bigger player.

At the same time, he noted, Republicans were questioning Obama’s decision to intervene militarily in Libya without seeking congressional authorization. And for all of the limits he has proposed in his new measure authorizing force against the Islamic State, Obama did not ask Congress to repeal a 2001 measure authorizing force against al-Qaida and its affiliates, which would mean he would still have wide discretion to wage war.

"The polarization that we’ve seen in every other sphere of American politics extends to interpretations of presidential power as well," Galston said. "It’s taken on new coloration in this era of endless war and boundless congressional dysfunction."

Gary J. Schmitt, a scholar at the American Enterprise Institute who was the executive director of a presidential intelligence board under Reagan, said one thing that had changed was the willingness of courts to involve themselves in what historically has been a tug of war left to the other two branches of government to resolve between themselves. In part, he said, that may reflect the changing challenges of recent years.

"In both these cases, the last two presidencies, it’s just a much more complex, interesting set of issues," Schmitt said.

But he said it would be better for courts to stay out and let the executive and legislative branches resolve it themselves.

"My large take," he said, "is that lawyers shouldn’t be allowed to play around with the Constitution."

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