Here is Ryan Goodman:
I have previously written that the 2001 authorization does not cover ISIS, and I noted: “As readers of Just Security, Lawfare, and Opinion Juris know, a remarkable consensus of opinion has emerged across our blogs that ISIS is not covered by the 2001 AUMF.”
Yet the White House ignored this remarkable academic consensus. Why? Well, remember the 2011 Libya war when the White House circumvented the War Powers Act by defining “hostilities” to exclude the act of raining down bombs and missiles on hostile troops? This broad interpretation of the 2001 AUMF is effectively a narrowing to nothing of the War Powers Act, henceforth, for all military activity directed against Islamic terrorists in the foreseeable future. Is it still possible to find this surprising?
The simple explanation is that in many settings–Libya, and, one supposes, this one–it is jointly in the interest of the president and relevant members of Congress to avoid a congressional vote that might force those members of Congress (specifically, Democrats right before an election) to go on record with a position that the party demands but their constituents reject.