At the White House on Thursday, Mr. Obama justified the airstrikes he had ordered in Iraq, saying, “It is not just part of my responsibility, but it is a sacred duty for me as commander in chief to protect the American people.” That is a blanket authority that lawmakers would be loath to give him for any extended campaign in Syria.

White House officials said its lawyers were exploring whether the 2001 Authorization for Use of Military Force could be retrofitted to justify strikes against ISIS in Iraq.