An excellent article exploring this issue about defining “assassination” is written by Jeffrey F. Addicott, a retired military officer and former senior legal advisor to the U.S. Army Special Forces, is an Assistant Professor of Law at St. Mary’s University-Law School in San Antonio, Texas, where he teaches a variety of courses on national security law.
http://jurist.law.pitt.edu/forum/forumnew68.php
You will note that Gerald Ford issued the first Executive Order banning political assassination in 1977. Reagan amended it in 1981. It hasn’t been changed since.