WASHINGTON — The White House is telling Congress that President Obama has the legal authority to continue American participation in the NATO-led air war in Libya, even though lawmakers have not authorized it.

In a broader package of materials the Obama administration is sending to Congress on Wednesday defending its Libya policy, the White House, for the first time, offers lawmakers and the public an argument for why Mr. Obama has not been violating the War Powers Resolution since May 20.

On that day, the Vietnam-era law’s 60-day deadline for terminating unauthorized hostilities appeared to pass. But the White House argued that the activities of United States military forces in Libya do not amount to full-blown “hostilities” at the level necessary to involve the section of the War Powers Resolution that imposes the deadline.

“We are acting lawfully,” said Harold Koh, the State Department legal adviser, who expanded on the administration’s reasoning in a joint interview with White House Counsel Robert Bauer.

The two senior administration lawyers contended that American forces have not been in “hostilities” at least since April 7, when NATO took over leadership in maintaining a no-flight zone in Libya, and the United States took up what is mainly a supporting role — providing surveillance and refueling for allied warplanes — although unmanned drones operated by the United States periodically fire missiles as well.

They argued that United States forces are at little risk in the operation because there are no American troops on the ground and Libyan forces are unable to exchange meaningful fire with American forces. They said that there was little risk of the military mission escalating, because it is constrained by the United Nations Security Council resolution that authorized use of air power to defend civilians.

“We are not saying the president can take the country into war on his own,” Mr. Koh said. “We are not saying the War Powers Resolution is unconstitutional or should be scrapped, or that we can refuse to consult Congress. We are saying the limited nature of this particular mission is not the kind of ‘hostilities’ envisioned by the War Powers Resolution.”

The administration unveiled its argument at a time when members of Congress have shown increasing skepticism about the Libya operation. On June 3, the House of Representatives passed a resolution declaring that the mission had not been authorized.

On Wednesday, the Speaker of the House, John Boehner, Republican of Ohio, sent Mr. Obama a letter pointing out that even under a flexible interpretation of War Powers Resolution that would allow hostilities to last 90 days without Congressional authorization, Mr. Obama was out of time. Mr. Boehner demanded a legal explanation by Friday.

“Given the mission you have ordered to the U.S. Armed Forces with respect to Libya and the text of the War Powers Resolution, the House is left to conclude that you have made one of two determinations: either you have concluded the War Powers Resolution does not apply to the mission in Libya, or you have determined the War Powers Resolution is contrary to the Constitution,” Mr. Boehner wrote. “The House, and the American people whom we represent, deserve to know the determination you have made.”

It remains to be seen whether majorities in Congress will accept the administration’s argument, defusing the confrontation, or whether the White House’s response will instead fuel greater criticism. Either way, because the War Powers Resolution does not include a definition of “hostilities” and the Supreme Court has never ruled on the issue, the legal debate is likely to be resolved politically, said Rick Pildes, a New York University law professor.

“There is no clear legal answer,” he said. “The president is taking a position, so the question is whether Congress accepts that position, or doesn’t accept that position and wants to insist that the operation can’t continue without affirmative authorization from Congress.”

Ten members of Congress — led by Rep. Dennis Kucinich, Democrat of Ohio, and Rep. Walter Jones, Republican of North Carolina — filed a lawsuit on Weednesday asking a judge to order Mr. Obama to stop the air war. The suit asserts that the operation is illegal because Congress did not authorize it. That lawsuit faces steep challenges, however, because courts in the past have dismissed similar cases on technical grounds.

The administration had earlier argued that Mr. Obama could initiate the intervention in Libya on his own authority as commander-in-chief because it was not a “war” in the constitutional sense. It also released a memorandum by the Justice Department’s Office of Legal Counsel agreeing that he could do so unilaterally because he anticipated that its nature, scope, and duration would be limited.
Since then, the conflict in Libya has dragged on longer than expected, and the goal of the NATO allies has all but openly shifted from merely defending civilians to forcing the Libyan dictator, Col. Muammar Qaddafi, from power. But Mr. Koh and Mr. Bauer said that while regime change in Libya may be a diplomatic goal, the military mission is separate, and remains limited to protecting civilians.
The administration legal team considered other approaches, including a proposal to stop the use of armed drones after May 20 in order to bolster the case that United States forces were no longer engaged in hostilities. But the White House ultimately decided not to make any changes in the military mission.
While many presidents have challenged the constitutionality of other aspects of the War Powers Resolution — which Congress enacted over President Nixon’s veto — no administration has said that the section imposing the 60-day clock was unconstitutional. In 1980,the Office of Legal Counsel concluded that it was within Congress’s constitutional power to enact such a limit on unauthorized hostilities.
Mr. Bauer and Mr. Koh said the 1980 memorandum remains in force, but that their legal argument does not invoke any constitutional challenge to the act.

It was not clear whether the Office of Legal Counsel has endorsed the White House’s interpretation of what “hostilities” means. Mr. Bauer declined to say whether the office had signed off on the theory, saying he would not discuss inter-agency deliberations.

Mr. Koh argued that the administration’s interpretation of the word was not unprecedented, noting that there have been previous disputes about whether the 60-day-clock portion of the War Powers Resolution applied to deployments where — unlike the Libya operation — there were troops on the ground and Americans suffered casualties.

Still, such previous cases typically involved peacekeeping missions in which the United States had been invited to take part, and there were only infrequent outbreaks of violence, like those in Lebanon, Somalia and Bosnia. Libya, by contrast, is an offensive mission involving sustained bombardment of a government’s forces.

The closest precedent was the NATO-led air war over Kosovo in 1999. In that case, the Clinton administration’s legal teamcharacterized the campaign, which involved many piloted American warplanes, as “hostilities” even though there was little exchange of fire from Serb forces after their air defenses were destroyed and there were no United States casualties.

In Kosovo, however, Congress appropriated specific funds for the mission before 60 days had passed. The Clinton administration decided that by providing the money, Congress had satisfied the requirements of the War Powers Resolution.