President Obama outlined a four-step plan on combating the threat of the Islamic State. Here are the highlights from his speech. (Nicki DeMarco and Ashleigh Joplin/The Washington Post)

Top lawyers at the Justice Department signed off on the White House’s conclusion that President Obama had sufficient legal authority to approve airstrikes in Syria, people familiar with the deliberations said Thursday, as the administration faced growing criticism for not first seeking congressional backing.

Obama aides consulted with lawyers at the Office of Legal Counsel in the lead-up to the president’s announcement Wednesday night that he was expanding U.S. military operations aimed at destroying the Islamic State, a radical Sunni group that is operating in Syria and Iraq. In a televised prime-time address, Obama said he had “the authority to address the threat” and did not need a formal vote on Capitol Hill.

It is unclear whether the Justice Department provided the White House with a detailed, written opinion or relayed its interpretation of the law orally.

Administration officials have declined to provide the administration’s legal rationale in detail, instead saying that Obama has the authority to strike al-Qaeda and associated groups under the 2001 Authorization for the Use of Military Force (AUMF), which Congress approved after the Sept. 11, 2001, attacks.

That assertion has been received skeptically by many experts in national-security law, in part because the Islamic State split from al-Qaeda and is considered a rival to the group.

Here are key moments from the speaker of the House's news conference following President Obama's speech on America's efforts to combat Islamic State militants. (AP)

Obama’s decision is based on a “novel” theory whose “premise is unconvincing,” Harvard Law School professor Jack Goldsmith wrote in a column on Time magazine’s Web site Thursday.

Goldsmith wrote that by loosely connecting the Islamic State to core al-Qaeda, the administration is claiming the authority to “use force endlessly against practically any ambitious jihadist terrorist group” that fights the United States. Obama’s gambit is “presidential unilateralism masquerading as implausible statutory interpretation,” Goldsmith wrote.

White House aides briefed lawmakers Thursday on the president’s strategy but failed to convince all of them of the legal rationale for circumventing Congress.

The administration “made the interesting argument that surely Congress didn’t intend to let al-Qaeda itself decide who should be covered,” said Rep. Adam B. Schiff (D-Calif.). “That’s true, but that’s a flaw in the AUMF. We also didn’t intend for it to be in effect 13 years later and apply it in places like Yemen or Somalia. I think the administration is being a bit selective in pointing to congressional intent.

In the case of Syria, Obama’s reliance on the 2001 legislation comes even though he called for its repeal last year, saying he wanted to work with Congress to refine the AUMF’s mandate.

“I will not sign laws designed to expand this mandate further,” he said in a speech at the National Defense University in May 2013. “Our systematic effort to dismantle terrorist organizations must continue. But this war, like all wars, must end.”

People who have been in close touch with the White House said the administration feared that the heightened political environment on Capitol Hill ahead of the midterm elections in November would make it virtually impossible to get approval for a new resolution authorizing strikes in Syria.

Some legal experts were sympathetic to the administration’s decision to bypass Congress and said Obama had the authority to act, even if al-Qaeda and the Islamic State are no longer aligned.

“He doesn’t need to carefully parse the organizational structure to find out who’s on the board of directors,” said Walter Dellinger, who headed the Justice Department’s Office of Legal Counsel under President Bill Clinton. “It is clear enough that this is a continuation of the morphing of the radical jihadist movement that was behind 9/11. And the authority of the president to use necessary and appropriate force has never been altered.

Indeed, some administration officials have stressed that the Islamic State was once known as al-Qaeda in Iraq. “This group is and has been al-Qaeda,” Secretary of State John F. Kerry told CNN in an interview Thursday. “By trying to change its name, it doesn’t change who it is, what it does.”

Last year, Obama asked Congress for approval to launch airstrikes in Syria after evidence that the government of President Bashar al-Assad had used chemical weapons against civilians. Obama pulled back after Congress indicated that it would not approve the strikes, and the administration subsequently negotiated an agreement with Russia, a traditional ally of Syria, to destroy the country’s chemical arsenal.

Obama also did not seek congressional approval before joining allies in the NATO bombing campaign in Libya in 2011.

In Syria this time, the only vote the White House has sought from Congress is one to approve a program to train and equip moderate rebels. Some legal experts said the decision not to seek congressional authorization for airstrikes in Syria erodes the balance of power between the executive and legislative branches.

“The more practice you have of a president considering Congress to be irrelevant to the decisions of war and peace, the more irrelevant Congress becomes,” said Benjamin Wittes, an expert on ­national-security law at the Brookings Institution.

Some lawmakers said that they hoped Congress would eventually vote on fresh authorization for the president’s military campaign.

Rep. Steny H. Hoyer (D-Md.) said Congress should take a “two-step process” — law­makers should first vote on a modest proposal that would allow Pentagon officials to train and arm moderate Syrian rebels. Later this year, a lame-duck session could provide time for expansive debate and resolutions providing clear language setting new parameters for U.S. military action in Syria.

“At some point in time, when we come back from the elections, I think there will be a consideration of a larger authorization for the use of force,” Hoyer said in an interview on C-SPAN’s “Newsmakers” that will air in full Sunday.

Paul Kane and Greg Miller contributed to this report.