Mr. Obama has argued that he already has the authority he needs to wage the conflict against the Islamic State under the authorization to fight the perpetrators of the terrorist attacks of Sept. 11, 2001, enacted by Congress shortly after the attacks.

That argument is controversial because the Islamic State is at odds with the leadership of Al Qaeda and its affiliate in Syria, the Nusra Front. Critics contend that the administration is stretching the Sept. 11 authorization too far by applying it to an organization that did not exist in 2001 and that operates far from Afghanistan.

The administration has countered that its position is legitimate because the Islamic State used to be a Qaeda affiliate in Iraq during the Iraq war. In an April 2015 speech, Stephen Preston, then the top Pentagon lawyer, argued that the fact that Al Qaeda splintered after the death of Osama bin Laden did not mean that the authority to keep fighting each successor faction came to an end.

Administration officials have also said the fight against the Islamic State is covered separately by the 2002 authorization President George W. Bush obtained from Congress for the invasion of Iraq, although they are not relying on it.

Image Capt. Nathan Michael Smith Credit... U.S. Army

The administration has asked Congress to enact new authorization for using military force against the Islamic State, but lawmakers have not acted on that request. They have, however, passed military appropriations bills that earmark funds for the effort against the Islamic State, which could suggest that lawmakers have acquiesced to the executive branch’s theory.