What is his legal argument?

Vicki Divoll, a former general counsel to the Senate Select Committee on Intelligence and former deputy legal adviser to the C.I.A.’s Counterterrorism Center, asks for Obama to release the legal basis for his use of drones to kill Americans without a trial.  She chides the Senate for not pushing on this, as they did with Mr. Gonzales of the GW administration.  She reminds us of 2008 candidate Obama's talk about transparency in government and his release of GW's legal memos, which were found to be faulty.  Is Obama worried that his legal basis for murder is also faulty?